This add-on implements simple workaround for Bug 453455. It will press OK button quickly upon "unknown content type dialog" appears while download attempt of known file type.
How to let it work:
1. Select "Open with" item at annoying "unknown content type dialog".
2. Choose proper application from dropdown "Open with" list.
3. Click "Force" button (it's available since version 0.80). It will start downloading and add an extension of current file into allowed extensions list for further automatic handling. Or you may add specific extensions into add-on "Options" manually in advance (for add-on versions below 0.80).
To disable specific extension handling (stop OK button autoclick for it) you have to open "Options" (menu "Tools\Add-ons", then choose "Extensions" tab and find this add-on) and remove specific extension from the list.