Note for readers from the future: the snap peeks at the first few bytes of the file to determine if it’s a .zip or .7z file. It doesn’t look at the file extension. Those are standard values and it’s not an exaggeration to say that if those bytes aren’t set then it’s not a .zip or .7z file. Since some applications that support additional formats will quietly use the correct decoder it’s possible that you think you have a .zip file but it’s actually something else.
When in doubt it’s best to create an empty archive, or one with just a single entry, and attach it your question. That allows us to verify how the third party library we use is identifying the file. There are some proprietary extensions the zip format, e.g., to support strong encryption, and it’s possible that the library is returning an unrecognized MIME type. We won’t be able to read the encrypted entries but we could provide a more meaningful error message in this case.