Work through this in order
- Compare the reported extension with known header signatures
- Check whether an archive is a renamed OOXML file
- Look for SQLite or PDF magic bytes
- Inspect the first 256 bytes when detection is inconclusive
- Treat uncertain results as unknown rather than guessing
Use the matching tool
Inspect a local copy with File Signature Detector. After the check, save or repair files only using trusted software that supports the format.