As a teacher, go to a course. Open the gradebook, click on import. Be sure to be in the CSV import type. Try to import any file who are not a text file, ex JPG, PDF, XLS etc. a warning display dialog should appear preventing you to upload this file. Try to import a CSV file. It should work. Try again with a txt file or any file with the mime type: text/plain (see function get_mimetypes_array() in filelib.php). It should work too. Go to the XML import type. Try to import a XML file. It should work. Try to import any other file type. You should see a warning display message preventing you to upload this file.