CURLRequest multipart File - nothing send

I did add an key for my URL and I where able to see everything I posted. form_params works flawlessly, but as the post where about sending a file and multipart, my answer reflects my findings with multipart.

And the only time I could get webhook.site to register content with multipart where when I submitted this array. If I only posted 'text' without any file, the content-length resulted to 0. And thus YMCA never where displayed (submitted?). And it only worked with PHP 7.4.

PHP Code:

$post_data = [
    'text' => 'YMCA',
    'file' => new \CURLFile(FCPATH.'robots.txt', 'text/plain', 'robots.txt')
]; 


okay, so it seems to be a Problem with the PHP Version? I only have 7.3.10 on my server sadly. Any Idea how to fix it or an alternative for a post multipart request?

PS: tested the above on my server with php 7.3 and you are right, just doesen't work