diff --git a/go.mod b/go.mod
index 04242dab..7075fa31 100644
--- a/go.mod
+++ b/go.mod
@@ -25,11 +25,13 @@ require (
 	github.com/spf13/jwalterweatherman v1.1.0 // indirect
 	github.com/spf13/pflag v1.0.5
 	github.com/spf13/viper v1.6.1
+	github.com/stretchr/testify v1.6.1
 	github.com/tomasen/realip v0.0.0-20180522021738-f0c99a92ddce
 	github.com/vmihailenco/msgpack v4.0.4+incompatible // indirect
 	github.com/xi2/xz v0.0.0-20171230120015-48954b6210f8 // indirect
 	go.etcd.io/bbolt v1.3.3
 	golang.org/x/crypto v0.0.0-20200510223506-06a226fb4e37
+	golang.org/x/image v0.0.0-20191009234506-e7c1f5e7dbb8
 	golang.org/x/net v0.0.0-20200528225125-3c3fba18258b // indirect
 	golang.org/x/sys v0.0.0-20200523222454-059865788121 // indirect
 	golang.org/x/text v0.3.2 // indirect
diff --git a/go.sum b/go.sum
index 36a4e5b6..7c4d8d6b 100644
--- a/go.sum
+++ b/go.sum
@@ -206,6 +206,8 @@ github.com/stretchr/testify v1.2.2 h1:bSDNvY7ZPG5RlJ8otE/7V6gMiyenm9RtJ7IUVIAoJ1
 github.com/stretchr/testify v1.2.2/go.mod h1:a8OnRcib4nhh0OaRAV+Yts87kKdq0PP7pXfy6kDkUVs=
 github.com/stretchr/testify v1.3.0 h1:TivCn/peBQ7UY8ooIcPgZFpTNSz0Q2U6UrFlUfqbe0Q=
 github.com/stretchr/testify v1.3.0/go.mod h1:M5WIy9Dh21IEIfnGCwXGc5bZfKNJtfHm1UVUgZn+9EI=
+github.com/stretchr/testify v1.6.1 h1:hDPOHmpOpP40lSULcqw7IrRb/u7w6RpDC9399XyoNd0=
+github.com/stretchr/testify v1.6.1/go.mod h1:6Fq8oRcR53rry900zMqJjRRixrwX3KX962/h/Wwjteg=
 github.com/subosito/gotenv v1.2.0 h1:Slr1R9HxAlEKefgq5jn9U+DnETlIUa6HfgEzj0g5d7s=
 github.com/subosito/gotenv v1.2.0/go.mod h1:N0PQaV/YGNqwC0u51sEeR/aUtSLEXKX9iv69rRypqCw=
 github.com/tmc/grpc-websocket-proxy v0.0.0-20190109142713-0ad062ec5ee5/go.mod h1:ncp9v5uamzpCO7NfCPTXjqaC+bZgJeR0sMTm6dMHP7U=
@@ -318,4 +320,6 @@ gopkg.in/yaml.v2 v2.2.4 h1:/eiJrUcujPVeJ3xlSWaiNi3uSVmDGBK1pDHUHAnao1I=
 gopkg.in/yaml.v2 v2.2.4/go.mod h1:hI93XBmqTisBFMUTm0b8Fm+jr3Dg1NNxqwp+5A1VGuI=
 gopkg.in/yaml.v2 v2.2.7 h1:VUgggvou5XRW9mHwD/yXxIYSMtY0zoKQf/v226p2nyo=
 gopkg.in/yaml.v2 v2.2.7/go.mod h1:hI93XBmqTisBFMUTm0b8Fm+jr3Dg1NNxqwp+5A1VGuI=
+gopkg.in/yaml.v3 v3.0.0-20200313102051-9f266ea9e77c h1:dUUwHk2QECo/6vqA44rthZ8ie2QXMNeKRTHCNY2nXvo=
+gopkg.in/yaml.v3 v3.0.0-20200313102051-9f266ea9e77c/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=
 honnef.co/go/tools v0.0.0-20190102054323-c2f93a96b099/go.mod h1:rf3lG4BRIbNafJWhAfAdb/ePZxsR/4RtNHQocxwk9r4=
diff --git a/http/preview.go b/http/preview.go
index c06ae436..0ad5e580 100644
--- a/http/preview.go
+++ b/http/preview.go
@@ -8,7 +8,6 @@ import (
 	"net/http"
 
 	"github.com/gorilla/mux"
-	"github.com/spf13/afero"
 
 	"github.com/filebrowser/filebrowser/v2/files"
 	"github.com/filebrowser/filebrowser/v2/img"
@@ -21,7 +20,7 @@ const (
 
 type ImgService interface {
 	FormatFromExtension(ext string) (img.Format, error)
-	Resize(ctx context.Context, file afero.File, width, height int, out io.Writer, options ...img.Option) error
+	Resize(ctx context.Context, in io.Reader, width, height int, out io.Writer, options ...img.Option) error
 }
 
 func previewHandler(imgSvc ImgService, enableThumbnails, resizePreview bool) handleFunc {
diff --git a/img/service.go b/img/service.go
index dc400a2e..1cb4fff7 100644
--- a/img/service.go
+++ b/img/service.go
@@ -2,14 +2,15 @@
 package img
 
 import (
+	"bytes"
 	"context"
 	"errors"
+	"fmt"
+	"image"
 	"io"
-	"path/filepath"
 
 	"github.com/disintegration/imaging"
 	"github.com/marusama/semaphore/v2"
-	"github.com/spf13/afero"
 )
 
 // ErrUnsupportedFormat means the given image format is not supported.
@@ -17,14 +18,12 @@ var ErrUnsupportedFormat = errors.New("unsupported image format")
 
 // Service
 type Service struct {
-	lowPrioritySem  semaphore.Semaphore
-	highPrioritySem semaphore.Semaphore
+	sem semaphore.Semaphore
 }
 
 func New(workers int) *Service {
 	return &Service{
-		lowPrioritySem:  semaphore.New(workers),
-		highPrioritySem: semaphore.New(workers),
+		sem: semaphore.New(workers),
 	}
 }
 
@@ -75,7 +74,7 @@ func (x Quality) resampleFilter() imaging.ResampleFilter {
 	case QualityLow:
 		return imaging.NearestNeighbor
 	default:
-		return imaging.Linear
+		return imaging.Box
 	}
 }
 
@@ -108,13 +107,19 @@ func (s *Service) FormatFromExtension(ext string) (Format, error) {
 }
 
 type resizeConfig struct {
-	prioritized bool
-	resizeMode  ResizeMode
-	quality     Quality
+	format     Format
+	resizeMode ResizeMode
+	quality    Quality
 }
 
 type Option func(*resizeConfig)
 
+func WithFormat(format Format) Option {
+	return func(config *resizeConfig) {
+		config.format = format
+	}
+}
+
 func WithMode(mode ResizeMode) Option {
 	return func(config *resizeConfig) {
 		config.resizeMode = mode
@@ -127,14 +132,19 @@ func WithQuality(quality Quality) Option {
 	}
 }
 
-func WithHighPriority() Option {
-	return func(config *resizeConfig) {
-		config.prioritized = true
+func (s *Service) Resize(ctx context.Context, in io.Reader, width, height int, out io.Writer, options ...Option) error {
+	if err := s.sem.Acquire(ctx, 1); err != nil {
+		return err
+	}
+	defer s.sem.Release(1)
+
+	format, wrappedReader, err := s.detectFormat(in)
+	if err != nil {
+		return err
 	}
-}
 
-func (s *Service) Resize(ctx context.Context, file afero.File, width, height int, out io.Writer, options ...Option) error {
 	config := resizeConfig{
+		format:     format,
 		resizeMode: ResizeModeFit,
 		quality:    QualityMedium,
 	}
@@ -142,21 +152,7 @@ func (s *Service) Resize(ctx context.Context, file afero.File, width, height int
 		option(&config)
 	}
 
-	sem := s.lowPrioritySem
-	if config.prioritized {
-		sem = s.highPrioritySem
-	}
-
-	if err := sem.Acquire(ctx, 1); err != nil {
-		return err
-	}
-	defer sem.Release(1)
-
-	format, err := s.FormatFromExtension(filepath.Ext(file.Name()))
-	if err != nil {
-		return ErrUnsupportedFormat
-	}
-	img, err := imaging.Decode(file, imaging.AutoOrientation(true))
+	img, err := imaging.Decode(wrappedReader, imaging.AutoOrientation(true))
 	if err != nil {
 		return err
 	}
@@ -168,5 +164,22 @@ func (s *Service) Resize(ctx context.Context, file afero.File, width, height int
 		img = imaging.Fit(img, width, height, config.quality.resampleFilter())
 	}
 
-	return imaging.Encode(out, img, format.toImaging())
+	return imaging.Encode(out, img, config.format.toImaging())
+}
+
+func (s *Service) detectFormat(in io.Reader) (Format, io.Reader, error) {
+	buf := &bytes.Buffer{}
+	r := io.TeeReader(in, buf)
+
+	_, imgFormat, err := image.DecodeConfig(r)
+	if err != nil {
+		return 0, nil, fmt.Errorf("%s: %w", err.Error(), ErrUnsupportedFormat)
+	}
+
+	format, err := ParseFormat(imgFormat)
+	if err != nil {
+		return 0, nil, ErrUnsupportedFormat
+	}
+
+	return format, io.MultiReader(buf, in), nil
 }
diff --git a/img/service_test.go b/img/service_test.go
new file mode 100644
index 00000000..eca36fb9
--- /dev/null
+++ b/img/service_test.go
@@ -0,0 +1,398 @@
+package img
+
+import (
+	"bytes"
+	"context"
+	"errors"
+	"image"
+	"image/gif"
+	"image/jpeg"
+	"image/png"
+	"io"
+	"testing"
+
+	"github.com/spf13/afero"
+	"github.com/stretchr/testify/require"
+	"golang.org/x/image/bmp"
+	"golang.org/x/image/tiff"
+)
+
+func TestService_Resize(t *testing.T) {
+	testCases := map[string]struct {
+		options []Option
+		width   int
+		height  int
+		source  func(t *testing.T) afero.File
+		matcher func(t *testing.T, reader io.Reader)
+		wantErr bool
+	}{
+		"fill upscale": {
+			options: []Option{WithMode(ResizeModeFill)},
+			width:   100,
+			height:  100,
+			source: func(t *testing.T) afero.File {
+				t.Helper()
+				return newGrayJpeg(t, 50, 20)
+			},
+			matcher: sizeMatcher(100, 100),
+		},
+		"fill downscale": {
+			options: []Option{WithMode(ResizeModeFill)},
+			width:   100,
+			height:  100,
+			source: func(t *testing.T) afero.File {
+				t.Helper()
+				return newGrayJpeg(t, 200, 150)
+			},
+			matcher: sizeMatcher(100, 100),
+		},
+		"fit upscale": {
+			options: []Option{WithMode(ResizeModeFit)},
+			width:   100,
+			height:  100,
+			source: func(t *testing.T) afero.File {
+				t.Helper()
+				return newGrayJpeg(t, 50, 20)
+			},
+			matcher: sizeMatcher(50, 20),
+		},
+		"fit downscale": {
+			options: []Option{WithMode(ResizeModeFit)},
+			width:   100,
+			height:  100,
+			source: func(t *testing.T) afero.File {
+				t.Helper()
+				return newGrayJpeg(t, 200, 150)
+			},
+			matcher: sizeMatcher(100, 75),
+		},
+		"keep original format": {
+			options: []Option{},
+			width:   100,
+			height:  100,
+			source: func(t *testing.T) afero.File {
+				t.Helper()
+				return newGrayPng(t, 200, 150)
+			},
+			matcher: formatMatcher(FormatPng),
+		},
+		"convert to jpeg": {
+			options: []Option{WithFormat(FormatJpeg)},
+			width:   100,
+			height:  100,
+			source: func(t *testing.T) afero.File {
+				t.Helper()
+				return newGrayJpeg(t, 200, 150)
+			},
+			matcher: formatMatcher(FormatJpeg),
+		},
+		"convert to png": {
+			options: []Option{WithFormat(FormatPng)},
+			width:   100,
+			height:  100,
+			source: func(t *testing.T) afero.File {
+				t.Helper()
+				return newGrayJpeg(t, 200, 150)
+			},
+			matcher: formatMatcher(FormatPng),
+		},
+		"convert to gif": {
+			options: []Option{WithFormat(FormatGif)},
+			width:   100,
+			height:  100,
+			source: func(t *testing.T) afero.File {
+				t.Helper()
+				return newGrayJpeg(t, 200, 150)
+			},
+			matcher: formatMatcher(FormatGif),
+		},
+		"convert to tiff": {
+			options: []Option{WithFormat(FormatTiff)},
+			width:   100,
+			height:  100,
+			source: func(t *testing.T) afero.File {
+				t.Helper()
+				return newGrayJpeg(t, 200, 150)
+			},
+			matcher: formatMatcher(FormatTiff),
+		},
+		"convert to bmp": {
+			options: []Option{WithFormat(FormatBmp)},
+			width:   100,
+			height:  100,
+			source: func(t *testing.T) afero.File {
+				t.Helper()
+				return newGrayJpeg(t, 200, 150)
+			},
+			matcher: formatMatcher(FormatBmp),
+		},
+		"convert to unknown": {
+			options: []Option{WithFormat(Format(-1))},
+			width:   100,
+			height:  100,
+			source: func(t *testing.T) afero.File {
+				t.Helper()
+				return newGrayJpeg(t, 200, 150)
+			},
+			matcher: formatMatcher(FormatJpeg),
+		},
+		"resize png": {
+			options: []Option{WithMode(ResizeModeFill)},
+			width:   100,
+			height:  100,
+			source: func(t *testing.T) afero.File {
+				t.Helper()
+				return newGrayPng(t, 200, 150)
+			},
+			matcher: sizeMatcher(100, 100),
+		},
+		"resize gif": {
+			options: []Option{WithMode(ResizeModeFill)},
+			width:   100,
+			height:  100,
+			source: func(t *testing.T) afero.File {
+				t.Helper()
+				return newGrayGif(t, 200, 150)
+			},
+			matcher: sizeMatcher(100, 100),
+		},
+		"resize tiff": {
+			options: []Option{WithMode(ResizeModeFill)},
+			width:   100,
+			height:  100,
+			source: func(t *testing.T) afero.File {
+				t.Helper()
+				return newGrayTiff(t, 200, 150)
+			},
+			matcher: sizeMatcher(100, 100),
+		},
+		"resize bmp": {
+			options: []Option{WithMode(ResizeModeFill)},
+			width:   100,
+			height:  100,
+			source: func(t *testing.T) afero.File {
+				t.Helper()
+				return newGrayBmp(t, 200, 150)
+			},
+			matcher: sizeMatcher(100, 100),
+		},
+		"resize with high quality": {
+			options: []Option{WithMode(ResizeModeFill), WithQuality(QualityHigh)},
+			width:   100,
+			height:  100,
+			source: func(t *testing.T) afero.File {
+				t.Helper()
+				return newGrayJpeg(t, 200, 150)
+			},
+			matcher: sizeMatcher(100, 100),
+		},
+		"resize with medium quality": {
+			options: []Option{WithMode(ResizeModeFill), WithQuality(QualityMedium)},
+			width:   100,
+			height:  100,
+			source: func(t *testing.T) afero.File {
+				t.Helper()
+				return newGrayJpeg(t, 200, 150)
+			},
+			matcher: sizeMatcher(100, 100),
+		},
+		"resize with low quality": {
+			options: []Option{WithMode(ResizeModeFill), WithQuality(QualityLow)},
+			width:   100,
+			height:  100,
+			source: func(t *testing.T) afero.File {
+				t.Helper()
+				return newGrayJpeg(t, 200, 150)
+			},
+			matcher: sizeMatcher(100, 100),
+		},
+		"resize with unknown quality": {
+			options: []Option{WithMode(ResizeModeFill), WithQuality(Quality(-1))},
+			width:   100,
+			height:  100,
+			source: func(t *testing.T) afero.File {
+				t.Helper()
+				return newGrayJpeg(t, 200, 150)
+			},
+			matcher: sizeMatcher(100, 100),
+		},
+		"broken file": {
+			options: []Option{WithMode(ResizeModeFit)},
+			width:   100,
+			height:  100,
+			source: func(t *testing.T) afero.File {
+				t.Helper()
+				fs := afero.NewMemMapFs()
+				file, err := fs.Create("image.jpg")
+				require.NoError(t, err)
+
+				_, err = file.WriteString("this is not an image")
+				require.NoError(t, err)
+
+				return file
+			},
+			wantErr: true,
+		},
+	}
+
+	for name, test := range testCases {
+		t.Run(name, func(t *testing.T) {
+			svc := New(1)
+			source := test.source(t)
+			defer source.Close()
+
+			buf := &bytes.Buffer{}
+			err := svc.Resize(context.Background(), source, test.width, test.height, buf, test.options...)
+			if (err != nil) != test.wantErr {
+				t.Fatalf("GetMarketSpecs() error = %v, wantErr %v", err, test.wantErr)
+			}
+			if err != nil {
+				return
+			}
+			test.matcher(t, buf)
+		})
+	}
+}
+
+func sizeMatcher(width, height int) func(t *testing.T, reader io.Reader) {
+	return func(t *testing.T, reader io.Reader) {
+		resizedImg, _, err := image.Decode(reader)
+		require.NoError(t, err)
+
+		require.Equal(t, width, resizedImg.Bounds().Dx())
+		require.Equal(t, height, resizedImg.Bounds().Dy())
+	}
+}
+
+func formatMatcher(format Format) func(t *testing.T, reader io.Reader) {
+	return func(t *testing.T, reader io.Reader) {
+		_, decodedFormat, err := image.DecodeConfig(reader)
+		require.NoError(t, err)
+
+		require.Equal(t, format.String(), decodedFormat)
+	}
+}
+
+func newGrayJpeg(t *testing.T, width, height int) afero.File {
+	fs := afero.NewMemMapFs()
+	file, err := fs.Create("image.jpg")
+	require.NoError(t, err)
+
+	img := image.NewGray(image.Rect(0, 0, width, height))
+	err = jpeg.Encode(file, img, &jpeg.Options{Quality: 90})
+	require.NoError(t, err)
+
+	_, err = file.Seek(0, io.SeekStart)
+	require.NoError(t, err)
+
+	return file
+}
+
+func newGrayPng(t *testing.T, width, height int) afero.File {
+	fs := afero.NewMemMapFs()
+	file, err := fs.Create("image.png")
+	require.NoError(t, err)
+
+	img := image.NewGray(image.Rect(0, 0, width, height))
+	err = png.Encode(file, img)
+	require.NoError(t, err)
+
+	_, err = file.Seek(0, io.SeekStart)
+	require.NoError(t, err)
+
+	return file
+}
+
+func newGrayGif(t *testing.T, width, height int) afero.File {
+	fs := afero.NewMemMapFs()
+	file, err := fs.Create("image.gif")
+	require.NoError(t, err)
+
+	img := image.NewGray(image.Rect(0, 0, width, height))
+	err = gif.Encode(file, img, nil)
+	require.NoError(t, err)
+
+	_, err = file.Seek(0, io.SeekStart)
+	require.NoError(t, err)
+
+	return file
+}
+
+func newGrayTiff(t *testing.T, width, height int) afero.File {
+	fs := afero.NewMemMapFs()
+	file, err := fs.Create("image.tiff")
+	require.NoError(t, err)
+
+	img := image.NewGray(image.Rect(0, 0, width, height))
+	err = tiff.Encode(file, img, nil)
+	require.NoError(t, err)
+
+	_, err = file.Seek(0, io.SeekStart)
+	require.NoError(t, err)
+
+	return file
+}
+
+func newGrayBmp(t *testing.T, width, height int) afero.File {
+	fs := afero.NewMemMapFs()
+	file, err := fs.Create("image.bmp")
+	require.NoError(t, err)
+
+	img := image.NewGray(image.Rect(0, 0, width, height))
+	err = bmp.Encode(file, img)
+	require.NoError(t, err)
+
+	_, err = file.Seek(0, io.SeekStart)
+	require.NoError(t, err)
+
+	return file
+}
+
+func TestService_FormatFromExtension(t *testing.T) {
+	testCases := map[string]struct {
+		ext     string
+		want    Format
+		wantErr error
+	}{
+		"jpg": {
+			ext:  ".jpg",
+			want: FormatJpeg,
+		},
+		"jpeg": {
+			ext:  ".jpeg",
+			want: FormatJpeg,
+		},
+		"png": {
+			ext:  ".png",
+			want: FormatPng,
+		},
+		"gif": {
+			ext:  ".gif",
+			want: FormatGif,
+		},
+		"tiff": {
+			ext:  ".tiff",
+			want: FormatTiff,
+		},
+		"bmp": {
+			ext:  ".bmp",
+			want: FormatBmp,
+		},
+		"unknown": {
+			ext:     ".mov",
+			wantErr: ErrUnsupportedFormat,
+		},
+	}
+
+	for name, test := range testCases {
+		t.Run(name, func(t *testing.T) {
+			svc := New(1)
+			got, err := svc.FormatFromExtension(test.ext)
+			require.Truef(t, errors.Is(err, test.wantErr), "error = %v, wantErr %v", err, test.wantErr)
+			if err != nil {
+				return
+			}
+			require.Equal(t, test.want, got)
+		})
+	}
+}