Right now we don't have any automated test coverage for the file upload dialog code (used for <input type="file">) and the required callbacks are not implemented in the WebView test browser so it's not easy to test it manually either.

We should definitely add support to the test browser, and also look at whether there's any automated tests possible. See  issue 780826  for an example of a bug in this code that ideally would have been caught by tests.

Assigning to Tim for now to take a look at supporting this in the test shell at least.