Source: Open & download attachments in Gmail – Computer – Gmail Help

 

  1. Check that you’re using a supported browser.
  2. Try turning off extensions you have on your browser one at a time.
  3. Clear your browser’s cache and cookies.
  4. Try opening Gmail with labs disabled at this page https://mail.google.com/mail/u/0/?labs=0. If this fixes the problem, try enabling labs one at a time.