Fix missing extension when saving a file without filters
Fix missing extension when saving a file without filters

Previously, when triggering the save dialog through e.g. `<a download>`
links (e.g. http://jsfiddle.net/koldev/cW7W5/), the extension was only
saved if Finder was set to show all extensions by default. We now always
display the extension to make sure that it is saved.

If we want to keep the extension hidden, we could also populate the
allowed file types array with the extension from the default filename,
but that would have interfered with how we set the filters.

the-ress/electron,electron/electron,seanchas116/electron,shiftkey/electron,gerhardberger/electron,gerhardberger/electron,seanchas116/electron,thomsonreuters/electron,bpasero/electron,thomsonreuters/electron,Floato/electron,gerhardberger/electron,bpasero/electron,electron/electron,bpasero/electron,electron/electron,shiftkey/electron,Floato/electron,the-ress/electron,gerhardberger/electron,electron/electron,thomsonreuters/electron,Floato/electron,bpasero/electron,the-ress/electron,gerhardberger/electron,seanchas116/electron,shiftkey/electron,seanchas116/electron,the-ress/electron,gerhardberger/electron,bpasero/electron,Floato/electron,gerhardberger/electron,seanchas116/electron,thomsonreuters/electron,electron/electron,shiftkey/electron,thomsonreuters/electron,Floato/electron,bpasero/electron,Floato/electron,the-ress/electron,electron/electron,seanchas116/electron,electron/electron,shiftkey/electron,thomsonreuters/electron,the-ress/electron,the-ress/electron,bpasero/electron,shiftkey/electron