Hi Francois, Thanks, but actually this still has issues. At first I thought it was clearly the simple thing I was missing, but then I remembered there's still a problem - the line in renderHead copied from the original MultiFileUploadField uses private members container, max, and upload, so the extending class won't be able to do this. I realize I could create my own versions of each of these (like you suggested with YOURJS), and then override every method that uses any of them, replacing the reference with the new versions. But that might propagate to usage of other fields, and at that point it might be simpler to just copy the entire contents of the original class's code into a new version of the class of my own, with the only modification being the location of the .js resource. Neither of these two things would be hard to do, and maybe they are simply the only solutions, but it feels like that's not great, because it loses the ability to leverage the fact that future versions of the api class might improve in other ways and there'd be maintenance issues keeping my version up-to-date with the rest of the api. Is that just what needs to be done, or is there possibly some other way to use the existing class directly and just swap some alternate resource file in place of the default one?