remote-viewer: add a default extension to screenshot filenames
    
    When doing a screenshot, if the user provides a filename without a file
    extension, an error occurs because the image format could not be determined.
    This patch adds a .png extension to such filenames, so that there is a default
    file format for screenshots.
    
    Fixes https://bugzilla.redhat.com/show_bug.cgi?id=1752514
    
    Reviewed-by: Victor Toso <victortoso@redhat.com>
    Signed-off-by: Julien Ropé <jrope@redhat.com>