Changing length of field mimetype to 255 chars.

According to
http://www.iana.org/assignments/media-types/media-types.xhtml
the length of mimetypes currently can be up to 85 chars, so with 255 we
will be safe.
Some minor translation corrections for de_DE.
Adding some more MIME types for ODF.

According to http://plan-b-for-openoffice.org/glossary/term/mime-type