I often upload a receipt to Dropbox straight from my iPhone camera. Knowing the purchase number I just create a new folder on Dropbox and upload the file there. The list of purchases usually indicates that there is an attachment (a paperclip icon by the purchase number). For receipts uploaded to Dropbox the paperclip never appears even though when I go to the purchase it's listed in the Files tab. It would be nice if all attachments were indicated by the paperclip.