fix: adapt image preview DoS protection to new FileStorage interface

File.File is now io.ReadCloser (no Seek). Buffer the file bytes
once via io.ReadAll, then use bytes.NewReader for both DecodeConfig
and Decode. Test updated to use io.NopCloser instead of afero.

pkg/models/task_attachment.go

@@ -222,10 +222,17 @@ func (ta *TaskAttachment) GetPreview(previewSize PreviewSize) []byte {
 	cacheKey := cacheKeyForTaskAttachmentPreview(ta.ID, previewSize)
 
 	result, err := keyvalue.Remember(cacheKey, func() (any, error) {
+		// Read all bytes up front so we can inspect dimensions without seeking.
+		// The file is an io.ReadCloser (no Seek), so we buffer it once.
+		data, err := io.ReadAll(ta.File.File)
+		if err != nil {
+			return nil, err
+		}
+
 		// Check image dimensions before full decode to prevent DoS
 		// from decompression bombs (small file, huge pixel dimensions)
 		const maxPixels = 50_000_000 // 50 megapixels
-		cfg, _, err := image.DecodeConfig(ta.File.File)
+		cfg, _, err := image.DecodeConfig(bytes.NewReader(data))
 		if err != nil {
 			return nil, err
 		}
@@ -233,12 +240,7 @@ func (ta *TaskAttachment) GetPreview(previewSize PreviewSize) []byte {
 			return nil, fmt.Errorf("image dimensions %dx%d exceed maximum of %d pixels", cfg.Width, cfg.Height, maxPixels)
 		}
 
-		// Seek back to start for full decode
-		if _, err := ta.File.File.Seek(0, io.SeekStart); err != nil {
-			return nil, err
-		}
-
-		img, _, err := image.Decode(ta.File.File)
+		img, _, err := image.Decode(bytes.NewReader(data))
 		if err != nil {
 			return nil, err
 		}

pkg/models/task_attachment_test.go

@@ -20,6 +20,7 @@ import (
 	"bytes"
 	"image"
 	"image/png"
+	"io"
 	"os"
 	"testing"
 
@@ -27,7 +28,6 @@ import (
 	"code.vikunja.io/api/pkg/files"
 	"code.vikunja.io/api/pkg/user"
 
-	"github.com/spf13/afero"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
@@ -195,18 +195,10 @@ func TestAttachmentPreviewRejectsLargeImages(t *testing.T) {
 	err := png.Encode(&buf, img)
 	require.NoError(t, err)
 
-	// Write the PNG to an in-memory afero filesystem so we get an afero.File
-	memFs := afero.NewMemMapFs()
-	err = afero.WriteFile(memFs, "large.png", buf.Bytes(), 0644)
-	require.NoError(t, err)
-	f, err := memFs.Open("large.png")
-	require.NoError(t, err)
-	defer f.Close()
-
 	attachment := &TaskAttachment{
 		ID: 999999,
 		File: &files.File{
-			File: f,
+			File: io.NopCloser(bytes.NewReader(buf.Bytes())),
 		},
 	}