Add pdf extension to files selected without one

[pavelbraginskiy]

Fixes #1565.

If a file with no extension is selected for pdf export, automatically add a .PDF extension.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 2.16%. Comparing base (ac1c381) to head (d85dcd1).
Report is 4 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##             master   #1653      +/-   ##
===========================================
- Coverage      2.16%   2.16%   -0.01%     
  Complexity      209     209              
===========================================
  Files           268     268              
  Lines         30911   30914       +3     
  Branches       5286    5287       +1     
===========================================
  Hits            669     669              
- Misses        30085   30088       +3     
  Partials        157     157              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.