This issue is similar than T288526.

List of steps to reproduce

• Create a form with an “uploadable” field using the “default filename“ option
• Add or edit a page with this form
• Select a file to upload

What happens?
The predefined default filename gets replaced by the actual name of the file. The suggested filename is lost.

What should have happened instead?
The default filename should stay on the upload form and change only if I change it manually.

Software version, browser information
MediaWiki 1.35.5
PHP 7.3.31-1~deb10u1 (fpm-fcgi)
MariaDB 10.3.31-MariaDB-0+deb10u1
ICU 63.1
PageForms 5.3.4 (b9a4c1d) and master HEAD
Mozilla Firefox 91.7.0esr