dont show delete file button when outside conversations directory. fixes #2007

src/main/java/eu/siacs/conversations/ui/ConversationFragment.java

@@ -579,11 +579,14 @@ public class ConversationFragment extends Fragment implements EditMessage.Keyboa
 				cancelTransmission.setVisible(true);
 			}
 			if (treatAsFile) {
+				String path = m.getRelativeFilePath();
+				if (path == null || !path.startsWith("/")) {
 					deleteFile.setVisible(true);
 					deleteFile.setTitle(activity.getString(R.string.delete_x_file, UIHelper.getFileDescriptionString(activity, m)));
 				}
 			}
 		}
+	}
 
 	@Override
 	public boolean onContextItemSelected(MenuItem item) {