I am working on file formats conversion. Some of them can be converted back and forth to others without losing any information (fields or precision of the numbers encoded), others file format do not take into account some data or meta-data or use approximated values. I was looking for a term to describe the first case, where two formats describe the same amount of information. I'm aware of the term isomorph that seems to describe the case quite well but I couldn't find any reference to any isomorph file format. Is there any other term used for describing the case?
I think that many programmers would look at it in terms of the conversions between the two formats, and say that each format can be converted to the other losslessly, and that it's possible to make a round-trip without losing any data. It's not a standard usage, but if you said that the two formats were round-trippable wrt each other, you would probably be understood. That said, you should still explain what that means!
Isomorphic file formats implies semantically that the formats have analogous data structures but no necessarily the same fidelity.
Perhaps this what you want to say: two file formats that have the same (data) fidelity.