What is the correct MIME type for a tar.gz file?

As of August 2012, the MIME type recommended in RFC 6713 is application/gzip.

According to the IANA registry, tar is not an official media type, so a GZipped tar file is officially only a compressed file.

Hypothetically, if a tarball were an official media type and following conventions, its MIME type would be application/tar (file extension .tar) and its compressed version would be application/tar+gzip (file extensions .tar.gz and .tgz).

Although most are deprecated, they are all technically correct, just different MIME types.

The correct MIME type is application/x-gzip according to cPanel standards.