An internet MIME-message is usually accompanied by a content-type string like text/plain or application/collection+json. The mimes library allows parsing these and can if types are subsets of another -- a parser that handles application/json, for example, will be perfectly fine handing the collection+json type with less semantics.
It also parses more complex strings including parameters (e.g. application/vnd.company.someapi; version=3) and can construct these as well.
See the docs for details.