WIP: upload CSV file for cohorting students

I don't think that adding "valid" here will work. We have utility file upload code for which you specify the allowed extensions. If a file is uploaded with a different extension, the generic error message you see here is returned.

So for instance, I don't think you would want "The file must be a valid .jpg file.".

To see the generic error messages, look at https://github.com/edx/edx-platform/pull/5961/files#diff-809f96adf264931867e3c547cf32ae38R56