Failed to open file chooser when trying to save with a SVG opened

[UbuntuOne]

1. Open a SVG file in Pinta.
2. File -> Save As...

```
$ pinta

(pinta:32488): GdkPixbuf-CRITICAL **: 13:20:44.867: gdk_pixbuf_calculate_rowstride: assertion 'width > 0' failed

(pinta:32488): GdkPixbuf-CRITICAL **: 13:20:44.867: gdk_pixbuf_calculate_rowstride: assertion 'width > 0' failed

(pinta:32488): GLib-GIO-WARNING **: 13:20:49.650: Error creating IO channel for /proc/self/mountinfo: Permission denied (g-file-error-quark, 2)

(pinta:32488): GLib-GIO-WARNING **: 13:20:58.229: Error creating IO channel for /proc/self/mountinfo: Permission denied (g-file-error-quark, 2)

(pinta:32488): Gtk-WARNING **: 13:20:58.321: Can't open portal file chooser: GDBus.Error:org.freedesktop.portal.Error.InvalidArgument: current_filter, if specified, must be present in filters list if list is nonempty
```

[Cameron White (cameronwhite91)]

I can reproduce this only with snap / flatpak packages. The file chooser portal seems to error out if the file filter isn't set to a valid filter, which in this case happens because SVG files can be read but can't be written to

[Cameron White (cameronwhite91)]

Fixed by https://github.com/PintaProject/Pinta/commit/404246ddbe47ef0a9b1a4ee7b17097fed2ade8cd