tarball: Try to guess archive format from mime type as last resort
Some archive files do not have any extension but we can guess their format by getting their mime types.
So add a last resort processing step to get archive format from mime type prior trying to extract it.
This will fix all these opam loader related errors.
Related to swh/infra/sysadm-environment#3468 (closed)
Migrated from D6301 (view on Phabricator)