The sample code you've posted doesn't do anything with the file's contents; it just reads them, puts them in a variable, then returns it as the result of dl_user. Perhaps it's printed elsewhere, but if not that's part of your problem.

The usual content type for arbitrary binary data is application/octet-stream, although I don't think that's part of your current problem.

It can also be awkward to read binary files with the diamond operator <FILE>, since line breaks don't occur in any particular place. It's usually best to use read, but again I don't think this is causing the problem you're seeing.