backport: when no extension is set, assume png (fix the previous check)

reported by Fathi Boudra

svn path=/branches/KDE/4.1/kdegraphics/okular/; revision=866615

ui/pageview.cpp

@@ -2001,7 +2001,7 @@ void PageView::contentsMouseReleaseEvent( QMouseEvent * e )
                     {
                         KMimeType::Ptr mime = KMimeType::findByUrl( fileName );
                         QString type;
-                        if ( !mime )
+                        if ( !mime || mime == KMimeType::defaultMimeTypePtr() )
                             type = "PNG";
                         else
                             type = mime->name().section( '/', -1 ).toUpper();