application/x-tar

The Unix tape archiver, still doing the bundling half of every .tar.gz fifty years on — tar concatenates files with 512-byte headers and does no compression at all, which is why it always arrives wrapped in gzip, bzip2, xz or zstd. The x- prefix is honest: the type was never registered with IANA, it is simply what everything sends. Sniffing tar is awkward — the original V7 format has no magic bytes whatsoever, and the POSIX ustar marker sits at octet offset 257, far deeper than the leading bytes most sniffers (and this site's signature table) look at.

A compressed tarball is served as the compressor's type, not this one — .tar.gz is application/gzip on the wire.

Defined by POSIX.1-2017 — pax (ustar interchange format). Registry facts from the IANA media-types registry via mime-db.