FileDialog.WILDCARD_PY doesn't append .py file extension on Linux

Example Code to reproduce issue -

``` python
from pyface.api import FileDialog, OK

def export():
dialog = FileDialog(
action='save as', wildcard=FileDialog.WILDCARD_PY
)
if dialog.open() != OK:
return
else:
filename = dialog.path

with open(filename, 'w') as f:
f.write('art.vandalay')

if __name__ == "__main__":
export()
```

Removing the wildcard argument prevents the addition of `.py` extension on all OS but it's presence doesn't seem to make any difference on Linux (Ubuntu 16.04)