Attaching documents to transactions is quite common use case for me, and it is a great feature.
But there are times I want to check all attachments related to a particular type of transaction. Allowing to add existing attachments to folders would be a great feature. This way the same attachment is re-used in the transaction & also in the folder. I can see it from both places.
Sample use-case: all my GST payment confirmations. These are mostly needed for audit / review purposes.
Currently I attach these confirmations to the payment when it is done, which adds more meaning to the payment transaction. But I also have to submit all these confirmations every once in a while for review. Currently I maintain a separate folder in my desktop to do this.
Overall, attachments are really good, but a way to group existing attachments would be cool.