ODF_3: Enforce mimetype entry

[carlwilson]

An ODF package MUST have a mimetype entry as specified in the Section 3.3 of the ODF specification v1.3.

A MIME type declaration is required in the first line of the file mimetype and an entry in the file \META-INF\manifest.xml

The MIME type must be declared. This is to ensure file format identification tools can accurately identify the file.
A media type (also known as a Multipurpose Internet Mail Extensions or MIME type) indicates the nature and format of a document, file, or assortment of bytes. MIME types are defined and standardized in IETF's RFC 6838.

If the MIME type is missing or not valid then the archive will be rejected.