FileChooser: Properly preselect MIME filter passed as "current_filter"

Other than the 'KFileWidget::setFilter' case,
calling 'KFileWidget::setMimeFilter' does not select the first filter
in the list, but takes an explicit (optional) parameter which of the
filters to preselect, "otherwise, a first item showing all the mimetypes
will be created". [1]

[1] https://api.kde.org/frameworks/kio/html/classKFileWidget.html#acdb884df2e08aa11c92adc1f96b6b2c1