Can't "save as" with non-standard file extension

Testing with both Qt Creator 3.3 and snapshot qt-creator-opensource-windows-x86-4.0.0.exe:

When I choose "Save as", set the file extension to something not in the list of supported extensions (in this case ".particle") and choose "All files " as the file type, the first file type in the filter list will be automatically selected.

In Qt Creator 3.3 it adds ".asm" giving "Foobar.particle.asm" and in Qt Creator 3.6.1 it adds ".3ds".