Dictionary mapping suffixes to suffixes. This is used to allow
recognition of encoded files for which the encoding and the type are
indicated by the same extension. For example, the .tgz
extension is mapped to .tar.gz to allow the encoding and type
to be recognized separately. This is initially a copy of the global
suffix_map defined in the module.
Dictionary mapping filename extensions to non-standard, but commonly
found MIME types. This is initially a copy of the global
common_types defined in the module.