A fully self hosted solution that for me is good enough and more easy is a WordPress site with the plugin WPforms (free version). No third party services used.
Update: not sure if the WPforms free version supports file uploads as the OP needs.
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This is imho wrong approach. Wordpress gives you extremely flexible possibilities but its hard to maintain. Like you really need to keep it updated or you will get hacked (every wp security hole gets hyper targeted). That means trust plugin authors they will keep up to date. WP is suited for smaller/mid businesses that keep it maintained but its not hands off.
Coding php/symfony form by hand (if you can do it) might be security through obscurity but realistically it will probably just work for a decade without issues or maintenance.