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.

Error was:  UPDATE "documents_documentversion" SET "document_id" = 25, "major" = 1, "minor" = 0, "micro" = 0, "release_level" = 1, "serial" = 0, "timestamp" = '2014-03-04 18:38:12.217915', "comment" = '', "file" = '91ffda55-4f8e-445d-a96f-9f7b83a50d7d', "mimetype" = 'application/vnd.openxmlform
ats-officedocument.wordprocessingml.document', "encoding" = 'binary', "filename" = 'Gamba.docx', "checksum" = '90f314277b70110a3e90b5e8fce286181dbb3f27224e4e770a11c8eb825631eb' WHERE "documents_documentversion"."id" = 25