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.
Merge pull request #75 from mbehrle/fix/v0.13.1

Changing length of field mimetype to 255 chars.