Use proper form widget extension instead of overriding the fileupload partial

[alxy]

As I can see, currently the entire fileupload form widget partial is being overridden by this plugin. This makes it very hard for other plugins to also add additional functionality to the fileupload widget.

Since some time now the title and description fields are actually a form widget which can be extended using the usual October extension methods. The idea would be to rather use this mechanism to extend the form with the required hidden fields. This would enable other plugins to also safely extend the form without colliding with this plugin.

[tobias-kuendig]

Thank you very much for this feedback @alxy! We will take a look at this!