Form.MULTIPART_HINT is not cleared correctly with nested forms

I have a page that looks like this

• Page
  • Form
    • Wizard (contains another Form)
      • StepOne - contains a FileUploadField
      • StepTwo - has no FileUploadField

(The outer form is there because I have some controls outside the wizard that needed to submit the wizard too).

Because I have been using Ajax ModalDialogs, it has been important to get Form.multipart set correctly (otherwise they don't submit back correctly). So I override Wizard.onActiveStepChanged and checked to see if the new wizard step contained a FileUploadField, and set Form.setMultiPart accordingly. However, this only clears Form.MULTIPART_HARD, Form.MULTIPART_HINT is left untouched.

When I step from StepOne to StepTwo, the inner form still has MULTIPART_HINT set as it contained a FileUploadField last render. Then the outer form's onRender fires, which clears MULTIPART_HINT, then the outer form's onComponentTag fires. When it checks isMultiPart(), it evaluates to true as the nested Form still returns true from its MULTIPART_HINT, and therefore tag.put("enctype", "multipart/form-data"); is executed. After that, the inner form's onRender fires which clears its MULTIPART_HINT.

When I try to submit StepTwo, the html has enctype="multipart/form-data", so it does a multipart submit. But wicket is not expecting a multipart submit so it doesn't find any of the submitted form data, and cannot determine which button is pressed, and loses all the form content.