R1:6df26b98c104

Comments: avoid confusing warning about "core:file"

Summary:
If you are reading this commit message, it probably happened to you this situation:

1. Somebody published a comment that shows the file F123
2. You Submit another comment that shows F123 again

After you click "Submit", a wild warning may appear:

> Post as Comment
> ----
> The action you are taking has no effect:
> Transaction (of type "core:file") has no effect.
> Do you want to post your comment anyway?
> [Undo] [Continue]

In this case you…