Map this record to a MIME type, or return null if it cannot be mapped.
Currently this method considers all
#TNF_MIME_MEDIA records to
be MIME records, as well as some
#TNF_WELL_KNOWN records such as
#RTD_TEXT. If this is a MIME record then the MIME type as string
is returned, otherwise null is returned.
This method does not perform validation that the MIME type is
actually valid. It always attempts to
return a string containing the type if this is a MIME record.
The returned MIME type will by normalized to lower-case using
Intent#normalizeMimeType.
The MIME payload can be obtained using
#getPayload.