submission: allow for inaccurate archive file extensions

[GraemeWatt]

The upload code relies on the file extension of an archive file (.zip, .tar, .tar.gz) to decide how to unpack it. But sometimes users try to upload files with the wrong extension, e.g. a .tar file that is gzipped or a .tar.gz file that is not gzipped. Currently a generic 500 error message is returned and the user often does not understand what went wrong. A more useful error message should be returned, or the code should accommodate possible mismatches between the file type and file extension.