Respond to CR.

SignalServiceKit/src/Messages/Attachments/TSAttachmentStream.m

@@ -347,6 +347,7 @@ NS_ASSUME_NONNULL_BEGIN
     }
 
     if (![[NSFileManager defaultManager] fileExistsAtPath:thumbnailPath]) {
+        // This isn't true for some useful edge cases tested by the Debug UI.
         DDLogError(@"%@ missing thumbnail for attachmentId: %@", self.logTag, self.uniqueId);
 
         return nil;