The cause of this is usually having special characters in the Filename such as dashes- slashes / \ etc. Try removing these from your file and reattaching the file to the Checklist Step.

This can be done in:
  1. From the menu in the top left select Enrollment Management
  2. Navigate to Admissions > Admissions Setup > Checklists
  3. Click edit next to the Checklist
  4. Click edit next to the Checklist Step for the Recommendation
  5. Click Delete
  6. Click Browse File (Cloud with an Arrow Icon)
  7. Select your File 
  8. Click Open
  9. Click Save & Close