As I don't see a point in comparing a mimetype with only a
substring, it probably could be a simple compareToAscii, but
play it save and use startsWith.

Change-Id: Ic33c8089283def615999ddf80cd12f6fe219ed17
Reviewed-on: https://gerrit.libreoffice.org/2907Reviewed-by: Thomas Arnhold <thomas@arnhold.org>
Reviewed-by: Fridrich Strba <fridrich@documentfoundation.org>
Tested-by: Fridrich Strba <fridrich@documentfoundation.org>