Hello Jens, for MPEG2 (and MPEG1) in principle any audio encoding that is allowed according to the specification should be accepted, so also “layer 3” (mp3). What is not accepted are encodings that are not part of the MPEG2 standard such as Dolby AC3, which is often used in camcorders. Files are also rejected sometimes if the header is not according to the standard. You are right though that the list requires further elaboration, but particularly for MPEG4, where we only accept a subset of what is allowed according to the spec. In any case, these type checker rules are indeed valid for the MPI archive and other archives may have different requirements. In such cases you can override the type checker decision in Arbil, as Twan mentioned. The legacy data remark refers to the fact that we do have such formats already in the archive but do not allow any further deposits of them.
The file type check in LAMUS can unfortunately only be done on the server side after upload, but checking in Arbil is of course local on your machine and should not take too long for a small number of files.