I've also tried to find a solution but the InputStream of the request can be read only once, hence I also think that WICKET-4715 should be reverted.

Maybe there is a way to have this feature - MultipartServletWebRequestImpl reads the servlet input stream in its constructor. If this logic is moved to some getXyz() method to make it lazy then I think it will work.

I've followed your advice making some refactoring and moving the code from constructor to a new method extractPostParameters(). It seems to work, although I didn't coded a use case test yet. However I don't like to check if extractPostParameters has been called in three different methods (getFiles, getFile and generatePostParameters).

Any better idea?

Reverted Wicket-4715, as this prevented Form to control the multipart file upload. Fixed small error in progressbar.js too.

Note that Servlet 3.0 makes non-file multipart parameters available via #getParameter() anyway.

Just for the record:
I've tried to delay parsing of the request multiparts until the parameters are accessed.
But before the form gets a hand on the request (to configure it with maxSize and uploadInfo), the request parameters will already be used by Wicket's request processing, thus already triggering the parsing.

If you find another solution, we can revisit this issue.