@lon2 can you post the full traceback?
Also, what is the website you are trying to use? Is the file input in a iframe?
I think there may be a few methods to use javascript to populate an input. The example site above uses iframes, and we views seem to have cross site scripting protections that prevent you from running JavaScript in iframes.
Also, it might be easier to use javascript to extract the action url, and populate the request directly, if there is a specific site you are targeting. Even if you can't find the url in the source, the browser obviously needs to know what it is, and it will be accessible via JavaScript.
By the way, in @mikael's webview, you can use the console() method to run a JavaScript console that is useful for debugging. (See the pull request to fix a bug on the JS logging script)