Fix media preview crash

Fixes #7050
// FREEBIE

src/org/thoughtcrime/securesms/mms/AttachmentManager.java

@@ -415,6 +415,7 @@ public class AttachmentManager {
       Intent intent = new Intent(context, MediaPreviewActivity.class);
       intent.addFlags(Intent.FLAG_GRANT_READ_URI_PERMISSION);
       intent.putExtra(MediaPreviewActivity.SIZE_EXTRA, slide.asAttachment().getSize());
+      intent.putExtra(MediaPreviewActivity.OUTGOING_EXTRA, true);
       intent.setDataAndType(slide.getUri(), slide.getContentType());
 
       context.startActivity(intent);