46_MIME_Type_check_for_file_upload
master
* adds unit tests for new methods in konova/tests/test_documents.py * refactors _accepted_mime_types of AbstractDocument into _valid_mime_types
* adds file mime type check on NewDocumentForm * adds file size check on NewDocumentForm * adds is_xy_valid methods as classmethods on AbstractDocument * adds translations for error messages * updates help text on NewDocumentForm file field