Thought I'd share with you an issue I discovered, with the hope that it'll help prevent any related frustration you may experience.
Filemaker 12's super cool new container field capabilities are awesome. Note, however, that if you use a calculation to designate the place where Filemaker will deposit the files that users attach to the database, the insert file function will fail if the data in any of the fields embodied within the calculation include a "/" symbol. Probably because Filemaker reads that symbol as a prompt to create another link in the filepath.
To circumvent this, I had to use "patterncount" in order to first determine whether any of the fields in my calculation contain that symbol. If the result is yes, Filemaker issues an alert to tell the user to re-enter the text without the symbol, and then returns the user to the original layout.
Hope this helps