Get mime type from download, if download is a file name

Get mime type from the download parameter instead of from the file, if
download is a file name. (added test for this too)

Use case: This helps when files are stored on disk as e.g. "4", "36" or "6d2fb3bfe024805b692280b65f9636ad" (as blobs basically for a database). The file name with suffix would then typically be taken from metadata in a database and added to the download with the download parameter.