Image MIME Reference
Generic-RAW
Digital Camera Raw (Generic Linux MIME)
image/x-dcraw
VS
Progressive JPEG (Legacy IE Alias)
image/pjpeg
PJPEG
A complete technical comparison of Digital Camera Raw (Generic Linux MIME) and Progressive JPEG (Legacy IE Alias) — covering compression, feature support, browser compatibility, magic bytes, and when to choose each format.
PJPEG wins on browser support
Feature Support
| Feature | Generic-RAW | PJPEG |
|---|---|---|
| Transparency (Alpha) | ✕ No | ✕ No |
| Animation Support | ✕ No | ✕ No |
| Progressive Loading | ✕ No | Yes |
| HDR Support | Yes | ✕ No |
| EXIF Metadata | Yes | Yes |
| ICC Color Profile | Yes | Yes |
Digital Camera Raw (Generic Linux MIME)
image/x-dcraw
Extension
.raw
Container
Varies (Predominantly TIFF/EP)
Compression
Lossless / Uncompressed
Algorithm
Varies (Often Lossless JPEG ITU-T81 or uncompressed)
Color Depth
12-bit, 14-bit, 16-bit
Developed by
freedesktop.org (MIME mapping) / Dave Coffin (dcraw tool)
Released
1997
Magic Bytes
Varies by vendor format
Progressive JPEG (Legacy IE Alias)
image/pjpeg
Extension
.jpeg.jpg.pjpeg.pjp.jfif
Container
JPEG (ISO/IEC 10918-1)
Compression
Lossy (DCT)
Algorithm
Discrete Cosine Transform (DCT)
Color Depth
8-bit, 12-bit
Developed by
Joint Photographic Experts Group / Microsoft (Alias)
Released
1992
Magic Bytes
FF D8 FF E0 (JFIF) / FF D8 FF E1 (Exif)
Browser Support Comparison
| Browser | Generic-RAW | PJPEG |
|---|---|---|
Chrome
|
✕ |
✓ All
|
Firefox
|
✕ |
✓ All
|
Safari
|
✕ |
✓ All
|
Edge
|
✕ |
✓ All
|
IE (Legacy)
|
✕ |
✓
|
Generic-RAW
Generic-RAW Strengths
- Provides a unified, highly useful catch-all MIME type for application developers who just need to know if a file is a raw image, rather than tracking 50+ individual camera extensions
- Deeply integrated into the Linux desktop ecosystem, ensuring file manager thumbnails work seamlessly via tools like ufraw-thumbnailer
Limitations
- Not an official IANA standard
- Can cause conflicts where standard TIFF files are misidentified as raw camera data by over-aggressive MIME sniffers
- The underlying 'dcraw' tool was abandoned by its creator in 2018 (though its successor, LibRaw, continues to maintain the ecosystem)
PJPEG
PJPEG Strengths
- Progressive JPEGs (which this MIME type attempted to describe) provide excellent perceived loading speeds on slow connections by rendering a low-resolution preview first
- Files sent with this MIME type are fully backwards compatible with any standard JPEG decoder
Limitations
- It is a non-standard MIME type that violates IANA specifications
- Causes strict file upload validation scripts to fail if not explicitly whitelisted or mapped
- Creates unnecessary confusion, as the official image/jpeg MIME type already fully encompasses progressive encoding
When to choose which format
Generic-RAW
Use Generic-RAW when…
- Linux Desktop File Associations (GNOME/KDE)
- Thumbnail Generation for Raw Images (UFRaw/LibRaw)
- Server-side Image Processing Fallbacks
PJPEG
Use PJPEG when…
- Legacy Internet Explorer File Uploads
- Progressive Image Rendering (Historical context)