I'm going to check this on my 14.10 VM

I was able to compile and run this on my 14.10 VM (32 Bit). Looks good so far.
Please try to delete the out folder and rebuild with grunt build. That helped on my machine.
We can use the fileTypes parameter and set the filter with https://www.geany.org/manual/gtk/gtk/GtkFileChooser.html#gtk-file-chooser-set-filter. This sounds like a good use of it.

I still wasn't able to compile this on my 14.10 VM (32 Bit), even though I ran git clean and started the setup from ground up.

I'm also not too experienced with native code and that's why I honestly don't know how to implement fileTypes. It looks like right now, it's not even implemented on Windows, so the param is quite useless anyway. Also, I don't know how an array on the JavaScript site can be converted to an ExtensionString on the native site.

@ingorichter As you managed building it, could you just make sure that running showOpenDialog with an undefined title still shows some default title?

@marcelgerber let me check tomorrow if we have any occurrences where we pass fileTypes to this function. Otherwise we can omit the implementation. But if this isn't implemented on Windows, I guess that we are not using it anywhere. I think we shouldn't change the function signature, but add a comment to the functions to make clear, that there is currently no need to support this feature. I assume, that the original plan to implement this feature was driven by the observations what the open/save dialogs provide for each platform. Since you can have this file filter functionality on all supported platforms, the engineer who implemented this was preparing for a possible use of it.
I'm going to check the behavior with an undefined title argument.

Passing undefined or null for the title results in the default title Open. This is totally fine to move forward. Let's get the one comment resolved and then we can merge it.

Done. Ready for merge.

Thanks @marcelgerber. Merged.

Fwiw, here's the Trello card for missing fileTypes support on Windows: https://trello.com/c/9dtYWEUj/20-1-windows-open-dialog-should-respect-passed-in-file-type-filters-see-showopendialog-executeshowopendialog. Could be expanded to cover Linux, too...