mimetype (string): Format of the file. If XML, specific schema can be
included after a +. Example: "application/pdf"
release_ids (array of string identifiers): references to release entities
that this file represents a manifestation of. Note that a single file can
contain multiple release references (eg, a PDF containing a full issue with
many articles), and that a release will often have multiple files (differing
only by watermarks, or different digitizations of the same printed work, or
variant MIME/media types of the same published work).