Image MIME Reference
BMP
Windows Bitmap (Legacy Windows MIME Alias)
image/x-windows-bmp
VS
Progressive JPEG (Legacy IE Alias)
image/pjpeg
PJPEG
A complete technical comparison of Windows Bitmap (Legacy Windows MIME Alias) and Progressive JPEG (Legacy IE Alias) — covering compression, feature support, browser compatibility, magic bytes, and when to choose each format.
Equal browser support
Feature Support
| Feature | BMP | PJPEG |
|---|---|---|
| Transparency (Alpha) | Yes | ✕ No |
| Animation Support | ✕ No | ✕ No |
| Progressive Loading | ✕ No | Yes |
| HDR Support | ✕ No | ✕ No |
| EXIF Metadata | ✕ No | Yes |
| ICC Color Profile | Yes | Yes |
Windows Bitmap (Legacy Windows MIME Alias)
image/x-windows-bmp
Extension
.bmp.dib
Container
DIB (Device-Independent Bitmap)
Compression
Uncompressed / Run-Length Encoding (RLE)
Algorithm
NoneRLE4RLE8
Color Depth
1-bit (Monochrome), 4-bit (16 colors), 8-bit (256 colors), 16-bit (High Color), 24-bit (True Color), 32-bit (True Color with Alpha)
Developed by
Microsoft Corporation
Released
1985
Magic Bytes
42 4D
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 | BMP | PJPEG |
|---|---|---|
Chrome
|
✓ 1
|
✓ All
|
Firefox
|
✓ 1
|
✓ All
|
Safari
|
✓ 1
|
✓ All
|
Edge
|
✓ 12
|
✓ All
|
IE (Legacy)
|
✓
|
✓
|
BMP
BMP Strengths
- Universally supported by all major operating systems and modern web browsers
- Maintains perfect, lossless fidelity because pixel data is stored without lossy compression artifacts
Limitations
- Massive file size overhead due to a lack of modern compression algorithms
- Legacy MIME strings like 'image/x-windows-bmp' can trigger validation warnings in strict security or API environments if unhandled
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
BMP
Use BMP when…
- Legacy Windows Desktop Registry Associations
- Older Enterprise Intranet Applications
- Windows Graphics Archival
PJPEG
Use PJPEG when…
- Legacy Internet Explorer File Uploads
- Progressive Image Rendering (Historical context)