Read the container before the extension
Identify the bytes, not only the suffix. List the structures the file contains.
File Formats
Field guides to the formats behind everyday work: what they store, where they fail, and which replacement makes sense.
4 guides in this collection
START HERE
The field guide to file formats
A format is a set of tradeoffs, not a three-letter label. Learn to choose one by what must survive the trip.
8 min read →02
Know the container
03What a file extension really tells you
Extensions help operating systems choose an app, but they are not proof of a file's content or safety.
10 min read →Know the container
04Containers vs. codecs: the difference that fixes conversion confusion
MP4 is usually a container; H.264 is usually a codec. That distinction explains most playback surprises.
11 min read →Know the container
Archival formats and delivery formats serve different jobs
The file you preserve should not always be the file you email, embed, or publish.
10 min read →How to approach file formats
Inspect signatures, MIME type, container structure, and the receiving application's documented support.
Start with the source file's signature and container, then list the output features that must survive: pages, streams, metadata, editability, transparency, or embedded resources. A compatible extension is only the first check. The destination application must also understand the internal encoding, and the converted file should be verified there before the richer master is archived.
Choose a format by what must survive
Choose an archival master and delivery derivative. Test the derivative in the receiving software.
Format identification checklist
- 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.