You could archive a description of the file format alongside the files. Maybe a pseudocode implementation too, or actual code (although who knows which programming languages will exist 300 years from now).

Or the dreaded .doc and .xls that even Microsoft has problems with today.

The US Library of Congress recommends archiving data in SQLite databases, since it’s a simple, well-documented format, SQLite is public domain, and SQLite devs have promised to support it for a long time and retain backwards compatibility indefinitely.

CSV and TSV are okay too of course, but it’s often much easier to deal with large datasets if they’re in an actual database format.