Check an Image’s Real File Type
Find out what format an image really is, regardless of its file name, and see whether its extension or MIME type is misleading the apps you upload it to.
- On-device
- No sign-up
- Batch
- Free, no watermark
Drop images on the plate
or paste with Ctrl/⌘ V
Processed on this device
How it works
- 01Drop, choose or paste the images to check.
- 02Read the detected format, extension and MIME type in the report.
- 03Look for flagged mismatches and copy the details.
Why use this tool
Reads the actual bytes
The format is identified from the file signature at the start of the file, not from the name someone gave it.
Mismatches flagged clearly
The real format is compared with the extension and the type your browser reports, and any disagreement is highlighted.
Batch checking
Check a whole folder of downloads or exports at once to find the files with misleading names.
Supported formats
Use cases
Upload keeps getting rejected
A portal refuses your “.png” logo. The checker shows it is really a JPG that was renamed, which explains the error.
iPhone photos named JPG
Occasionally a photo ends in .jpg but actually contains HEIC data, which many Windows apps can’t open. Confirm the real format before converting.
Validating user uploads
Developers can see how a set of test files will look to server-side checks that compare the extension with the actual content.
Technical details
Most image formats begin with a fixed byte sequence called a magic number or file signature. A PNG starts with the bytes 89 50 4E 47, a JPG with FF D8 FF, and GIF, WebP, AVIF, HEIC, BMP, TIFF and ICO have their own signatures. The browser’s reported MIME type is normally guessed from the extension, so a renamed file gets the wrong type. Upload forms and image libraries that check the signature will then reject the file or fail to decode it.
Mismatches usually come from apps that save in one format but keep the old name, from downloads where a server sent a different format than the link suggested, or from manual renaming. Renaming a file never converts it. If you need a genuine PNG, run the file through the Image Converter, which decodes the real format and writes a proper new file that any validator will accept.
Your files stay with you
This tool runs entirely inside your browser using Web Workers and WebAssembly. Your images are never uploaded, never stored on a server and never used to train AI. Close the tab and they’re gone. Learn more
Frequently asked questions
Why does a website say my PNG is not a valid image?
The file is often a different format with a .png extension, for example a JPG or WebP that was renamed or saved by an app that used the wrong name. Sites that inspect the file’s bytes notice the mismatch. The checker shows the real format so you can convert it properly.
What is a MIME type?
A MIME type is a label such as image/jpeg or image/png that tells browsers and servers what kind of data a file holds. It is usually assigned from the file extension. When the extension is wrong, the MIME type is wrong too, even though the content is unchanged.
Can I fix the format by renaming the file?
Only if you rename it to match its real format, which the checker tells you. Renaming a JPG to .png does not make it a PNG; the bytes stay JPG. To get a different format, convert the image so it is actually re-encoded.
What are magic bytes?
Magic bytes are the first few bytes of a file that identify its format, like a signature. They are written by the program that created the file and don’t change when the file is renamed, which makes them a more reliable indicator than the extension.
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