CURLRequest multipart File - nothing send

I have made a couple of tests myself. I can't send any file with or without* CodeIgniter cURL class to webhook.site. Haven't found any resources that you should be able to send files to them or not.

Using PHP 7.3.12 I can't send anything with CodeIgniter, the same as you. But with PHP 7.4 it takes my "form values".
If I only append a string (not an CURLFile) CodeIgniter won't work with PHP 7.4 either. It will always be content-length: 0.

I can however send "form values" with cURL stand alone with both 7.3.12 and 7.4.

There aren't any solution in my post. Just wanted to say you aren't alone in this madness...

* Using curl stand alone.