If a student uploads an assignment then renames the file in the Moodle interface (removing the file extension) the document is not converted to pdf on the inline grading screen.
Steps to recreate the problem:
- Log in as a student
- Submit an assignment (myassignment.docx)
- Click the file in the 'File submissions' panel
- Remove the .docx file extension from the 'Name' box (image 1)
- Press 'Update'
- Log in as a teacher
- Visit the assignment and press 'Grade' for the student
- The review panel (left) is blank (image 2)
Students often use the 'Name' box as a title for their submission so it's quite common for students to remove the document file extension.