title: File API returns "invalid/octet-stream" for pdf file in Firefox
body: <p>I'm doing file upload which checks file mime type in browser. It's uses jQuery File Upload Plugin and works good everywhere except one user. </p>
  
  <p>For some reason file.type (html5 file api) returns "<strong>invalid/octet-stream</strong>" for pdf file (usually it's "application/pdf"). Pdf file is simple and can be opened at this pc without any problems.</p>
  
  <p>Environment: Windows 7 and Firefox 40. Not reproduced in chrome or explorer.</p>