Extensions help operating systems choose an app, but they are not proof of a file's content or safety. There is no one-line answer here: it depends on evidence from the source, the specific properties named below, and the destination's real requirements.
Inspect signatures, MIME type, container structure, and the receiving application's documented support. In “What a file extension really tells you,” begin with this specific observation: A MIME type and a file signature are stronger identification signals than the filename.
Three findings that guide this choice
The core distinction
A MIME type and a file signature are stronger identification signals than the filename.
The practical trade-off
Some families use several equivalent extensions, such as JPG and JPEG.
The verification test
A container extension may reveal little about the codec inside.
A practical decision for this workflow
Verify unexpected files with a trusted inspector before opening or converting them.
Use a container extension may reveal little about the codec inside. as the acceptance test on a representative source before committing an entire archive, publication, or delivery batch.
A common case is a photo saved with a misleading suffix: the JFIF to PNG route and converting iPhone HEIC photos to JPG both rewrite the real encoding, not just the name.
The file formats mistake to avoid here
Assuming every .docx or .pdf is harmless simply because the extension looks familiar.
That failure conflicts directly with the recommendation for “What a file extension really tells you”: Verify unexpected files with a trusted inspector before opening or converting them.
Extensions matter most when a file arrives from someone else: what to check before uploading a file covers the first steps, and compressed bundles are a frequent culprit, so the RAR archive converter and GZ file converter show what is really packed inside.
Review checklist before delivery
- ✓The file signature agrees with the extension.
- ✓The receiving app supports the container and internal encoding.
- ✓Required metadata or embedded resources remain present.
- ✓A richer master is stored separately from the delivery copy.
Judge “What a file extension really tells you” by whether the result passes these specific checks, not by whether a new file merely opens.