fix issue where file records without physical files were failing validation on assignment update/save
fix issue where file records without physical files were failing validation on assignment update/save

UM-USElab/gradecraft-development,mkoon/gradecraft-development,UM-USElab/gradecraft-development,UM-USElab/gradecraft-development,mkoon/gradecraft-development,mkoon/gradecraft-development,UM-USElab/gradecraft-development