Notify the user when the original file extension has been removed or modified upon file editing in the filepicker

An Admin/Teacher can define 'Accepted file types' (e.g. file types that are supported by the document converter) in the assignment's settings page under 'Submission types'. This would disable the Students to upload or modify a file extension to something that is not supported by the document converter

Test with both Boost and Classic themes.

1. Login as Admin.
2. Create a Course.
3. Enroll a Student in the Course.
4. Create an Assignment activity.
   1. For 'Submission types' enable 'File submissions'
5. Login as a Student.
6. Upload a file (e.g. .docx) and submit the assignment.
7. Go back to the Assignment.
8. Edit the assignment.
9. Click the file in the 'File submissions' panel
10. Remove the .docx file extension from the 'Name' input field
11. Press 'Update'
    1. make sure a pop up dialog is displayed and notifies the Student that the file extension has been modified during the file update process and request a confirmation for the update action.
    2. Verify the confirmation dialogue appears in the center of the screen (borrowed from MDL-68168).