The METS standard has a mimetype attribute on files "<file MIMETYPE="application/pdf" ... /> but this information is currently ignored by Rosetta during the file-type identification process on ingest.

Mimetye should be at least as reliable as file extensions (because mimetype is a standard, is actively maintained and widely use by a good portion of software for identifying file types).

In particular, recognising mimetypes is likely to greatly increase the ease of ingest of content from internet-facing content management systems where it is very widely used

See also https://tools.ietf.org/html/rfc6838