I've tried it both ways, and both times, I get the following message:
Server Error - The remote server returned an error: (404) Not Found.
For the create attachments way, they attached just fine within ArcMap, however, when opening them up in Flex, I get that message. I pulled the files from a subfolder within my www root folder.
The URL option did the same thing, and I made sure my URL was correct as well.
Thanks,
Braden