You might want to download a free trial copy of FIleMaker 11 and see for yourself. In the downloads link at the top of this screen and in your FileMaker 11 help menu are links to a PDF on IWP that you will want to read.
I believe you can download and view items from containers (if they're stored in the database itself), but I think you can't *add* an item to an empty container, is this correct?
From a web browser, you are correct that you can't upload, but you can also display store by reference images from containers if the files are located in the correct folder.
How can you tell (without getting in there and trying it) if a script is going to be web compatible?
Open the script editor and select Web Publishing from the Show Compatibility drop down. All incompatible steps will be shown in grey. You can also look up steps in FileMaker Help and their help entries include a place where you'll be able to see if it is web compatible or not.