WebsiteFeedbackForm::processImage() breaks image upload

Problem/Motivation

Image upload is not working.
When using the full form (/admin/content/website-feedback/add), the image is simply never uploaded.
When using the AJAX form, the form submit button uploads the image and never submits the form.

Steps to reproduce

Browse to /admin/content/website-feedback/add.
Fill the summary and image fields.
Submit the form: the image is not submitted.

Proposed resolution

The comment says AJAX upload is removed "for security reason" without more explanation.

Removing the processImage() method fixes the problem for us.

Remaining tasks

User interface changes

API changes

Data model changes