Netlify Forms doesn't send File Attachment

Hi,

in one of my sites I use Netlify Forms with a input type “file” as explained in the Netlify docs.
The problem is that most of the time the attachment is lost.
The form is this: Work with Us | De Robert Calzature

I’ve already added a check if the file is bigger than 1MB, and added accepted files (.doc, .docx and .pdf), but that didn’t solve the problem.

What can cause this?