I'm not sure why but I thought when I dragged a file onto a thought it would be moved into storage associated with the Brain itself. But it seems to be referring to the file as it exists on the file system. This could make sense for a document in, say, ~/Documents. But as an example of where this does not work I am a heavy user of DropZone on my Mac. If I drop a file from there the attachment gets a file reference like
/private/var/folders/dw/tmf07r7n1cv6khc7rc35q4cw0000gn/T/WebKitPDFs-dcEKU2/theshipitjournal.pdf Pretty soon that is going to pointing at nothing.
Personally although it might lead to some duplication I'd like all attached files to end up in storage associated with the brain. I can solve that problem later if it turns out to be relevant. I'd much rather that and know that any file referenced in my brain is reachable in my brain.
At the very least I'd like the option and Brain itself should check to see if a file reference is likely to be durable (I suspect a relatively simple heuristic would cover 80% of the problem cases).