Hi Dave. To help you on your way, we really need some more information about what type of file you have. A file extension in itself does not say much, everyone can choose any combination of letters.
In addition, not every formal file storage is also publicly known. Often a file starts with a fixed combination of characters, and sometimes there is something like the start and / or the length of the amount of data. Often the solution is to download the file as a random file access with the record length of one byte. But again, if you don't explain where you got the information from, it's just guesswork.