First, I really like this idea. Sort of a spotlight comments (if you are familiar with macOS) that are easier to access.
Currently when I do this I separate out the attachments into individual thoughts and make the note applicable to the attachment. The other workaround is referencing the attachment name and then add the note under it (I use a snippet autocomplete for this). However, these are just workarounds. The ability to add notes to attachments and make them searchable would allow for more information dense brains which I can see being useful in research, medical or other areas. Several uses I can think of straight off the bat: Yes, all of these can be done in the notes, but if the Thought notes are specific to the topic and there are multiple supporting documents, this muddies the water.
Attribution (URL the attachment was found) Commentary on research document specific to that document Page number references (for quick reference to specific parts of the document)