When you sign up for that service, a folder called Attachments is added to your Dropbox account. That's where the stuff goes.
AFAIK, the only thing that actually gets into the Dropbox attachment folder is an email with an attachment. There are settings in the SendToDropbox application to include the e-mail as either plain text or as html. I just ran a test and this is how mine works.
Note that each item is separate from the other: You get a text/html file of the e-mail message, then a separate file for each separate attachment. They are not linked together within the Attachments folder. What I do is go to Dropbox via my PC and create folders for each e-mail, just so I can keep them straight.
If the e-mail doesn't have an attachment - it won't show up.
Hope that helps.
Marilyn