fix resizing images in image viewer

apps/files_imageviewer/js/lightbox.js

@@ -36,7 +36,7 @@ function showLightbox(container,img){
 			img.width = maxHeight * ratio;
 		} else {
 			img.width = maxWidth;
-			img.height = maxWidth * ratio;
+			img.height = maxWidth / ratio;
 		}
 	}
 	container.empty();
@@ -55,4 +55,4 @@ function hideLightbox(){
 		$('#lightbox').hide();
 		lightBoxShown=false;
 	}
-}
\ No newline at end of file
+}