Image Format Checker: MIME and Metadata
Detect the real format of an image file — JPEG, PNG, WebP, GIF, AVIF — from magic bytes.
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Tags: image format checker, detect image format, image file type inspector
Image Format Checker: MIME and Metadata An image format checker reads the first bytes of a file and identifies the true format from binary signatures — regardless of the file extension. It also extracts metadata like dimensions, color depth, and MIME type. --- Why Not Trust File Extensions? File extensions are unreliable indicators of format: Users rename files (e.g., to ) Some tools save files with the wrong extension Malicious files may use a benign extension to evade filters Downloaded files sometimes lose their extension Magic bytes are authoritative. Every image format defines a fixed byte sequence at the start of the file that unambiguously identifies it. What is magic byte signatures? | Format | Hex signature | Offset | ASCII | |--------|--------------|--------|-------| | JPEG | |…
Frequently Asked Questions
How do I detect an image's real file format?
Read the first 8–16 bytes of the file and compare against known magic byte signatures. JPEG starts with FFD8FF, PNG with 89504E47, WebP with 52494646 (RIFF) followed by 57454250 (WEBP) at offset 8. This works regardless of the file extension.
What are magic bytes in image files?
Magic bytes are fixed byte sequences at the start of a file that identify its format. They're called 'magic' because they work like a file fingerprint — independent of the filename or extension. The `file(1)` command on Unix uses a database of magic byte patterns.
What is the difference between JPEG and JPG?
No difference in format — JPG is the 3-character extension from the DOS/Windows era (which limited extensions to 3 characters), while JPEG is the full name. Both have the same magic bytes (FFD8FF) and the same encoding. Modern systems accept both extensions.
How do I detect HEIC images?
HEIC files start with a 4-byte size field, then 'ftyp' at offset 4, followed by the brand. HEIC uses 'heic' or 'heix', HEIF uses 'heif' or 'mif1'. Read bytes 4–11 and check for these ASCII strings.
What is the MIME type for WebP?
The MIME type for WebP is `image/webp`. The file signature is RIFF (52 49 46 46) at offset 0, followed by the file size as a 4-byte little-endian integer, then WEBP (57 45 42 50) at offset 8.
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