set static naive JFIF headers for compatibility with kindle

internal/pkg/epubzip/image.go

@@ -17,6 +17,18 @@ type Image struct {
 	Data   []byte
 }
 
+var naiveJFIFHeader = []byte{
+	0xFF, 0xD8, // SOI
+	0xFF, 0xE0, // APP0 Marker
+	0x00, 0x10, // Length
+	0x4A, 0x46, 0x49, 0x46, 0x00, // JFIF\0
+	0x01, 0x02, // 1.02
+	0x00,       // Density type
+	0x00, 0x01, // X Density
+	0x00, 0x01, // Y Density
+	0x00, 0x00, // No Thumbnail
+}
+
 // CompressImage create gzip encoded jpeg
 func CompressImage(filename string, format string, img image.Image, quality int) (Image, error) {
 	var (
@@ -28,7 +40,11 @@ func CompressImage(filename string, format string, img image.Image, quality int)
 	case "png":
 		err = png.Encode(&data, img)
 	case "jpeg":
-		err = jpeg.Encode(&data, img, &jpeg.Options{Quality: quality})
+		// static JFIF header for better compatibility with Kindle devices
+		var b bytes.Buffer
+		err = jpeg.Encode(&b, img, &jpeg.Options{Quality: quality})
+		data.Write(naiveJFIFHeader)
+		data.Write(b.Bytes()[2:])
 	default:
 		err = fmt.Errorf("unknown format %q", format)
 	}