Unfortunately you cannot insert items into Container fields over Web Publishing using FileMaker Pro alone, however there is a plug-in you can use, created by 360Works and is called SuperContainer:
Very highly rated by other developers and definitely worth a test of the demo.
Also you can only insert one file into a single container, so you would need one record per file, which can be in a separate table and related to a main record to group them if you needed.
I hope this helps.
Wow, that 360 works installer is just atrocious.
First, it's a zip file.
Second, it's a structured zip file, so I can't just drag the contents out, I have to extract it.
Third, the installer is java based.
Fourth, now that I've installed java, it won't let me install it because it's looking for FM Pro Server, and I only downloaded the FM Pro trial.