Sets the current filter.
Only the files that pass the filter will be displayed. If the user-selectable list of filters is non-empty, then the filter should be one of the filters in that list.
Setting the current filter when the list of filters is empty is useful if you want to restrict the displayed set of files without letting the user change it.
| this |
a `GtkFileChooser` |
| filter |
a `GtkFileFilter` |