Better extension handling

[philstopford]

Oasis files may have .oasis as an extension. KLayout only seems to recognize .oas as a legitimate extension in its file dialogs. It won't show .oasis unless 'all files' are selected. It also defaults to saving GDSII when a .oasis file is selected for saving; the user has to manually change the file data to OASIS in the output options. KLayout may also then choose to append .GDS or .OAS to the file (myfile.oasis.GDS or myfile.oasis.OAS).

Can this behavior be improved? I could not find any preferences to add extensions or to change from the default-to-GDS behavior.