Step 1 / source
Choose the source
Drop one file here
Choose the file whose extension or reported type you need to sanity-check.
Checking publication status...LOCAL FILE INSPECTOR / TYPE SIGNALS
MIME Type Checker
Compare a file extension, the browser-provided MIME claim, and known leading-byte signatures in one downloadable report.
Processed in this browserOne file up to 2 GB. Only the first 8 KB is read; the signature catalogue is intentionally finite.
1 file - 8 KB inspected
No file upload
This operation runs with browser APIs. Nirmion receives catalogue requests, not the selected source bytes.
Original stays unchanged
The converter creates a separately named output. It never writes back into the source file.
Bounded work
One file up to 2 GB. Only the first 8 KB is read; the signature catalogue is intentionally finite.
Plain-language method
Why one type signal is not enough
Extensions are user-editable, browser MIME values can be empty, and signatures cover only known formats. The report shows all available signals instead of pretending any single one is absolute proof.
- 01Choose a valid source
Choose the exact file you plan to upload, serve, or convert. Large files are safe to inspect because only a small header slice is read.
- 02Review the conversion choices
There are no guesses to tune. Known signatures take priority, then extension mapping, then unknown.
- 03Inspect the separate result
Review the on-page JSON and optionally download the _nirmion_tools_mime_report.json file.
Useful when
Practical jobs this tool handles
- Investigate a file rejected by an upload form.
- Check whether an extension matches known bytes.
- Record basic type evidence for a support ticket.
Before converting
Treat the result as a signal
- A match does not prove the whole file is valid.
- An unknown result may simply mean the signature is not catalogued.
- Use specialist security tools for untrusted executables or malware analysis.
Know the boundary
Not a content or malware validator
The checker does not parse the complete format, verify embedded objects, or establish safety. A spoofed or polyglot file can need deeper analysis.
Quick answers
Questions before you convert
MIME inspection is useful when its confidence is understood.
Why is the browser MIME value blank?
Browsers often infer type from the extension and may provide no claim for uncommon formats.
What does mismatch mean?
Known signature bytes and the filename extension point to different format families.
Can I use this as an antivirus?
No. It identifies a bounded set of file signatures and performs no malware detection.