use same code that is used to open files for images

fixes #1323

src/main/java/eu/siacs/conversations/ui/adapter/MessageAdapter.java

@@ -395,10 +395,7 @@ public class MessageAdapter extends ArrayAdapter<Message> {
 
 			@Override
 			public void onClick(View v) {
-				Intent intent = new Intent(Intent.ACTION_VIEW);
-				intent.setDataAndType(activity.xmppConnectionService
-						.getFileBackend().getJingleFileUri(message), "image/*");
-				getContext().startActivity(intent);
+				openDownloadable(message);
 			}
 		});
 		viewHolder.image.setOnLongClickListener(openContextMenu);