I usually use this page https://microsoftedgesupport.microsoft.com/hc/en-us/requests/new here to submit feedback but it has been few days that It no longer lets me to do that. the box for select and upload files only appears for 1 milisecond and after that disappears. I only see subject, email address, description boxes, the place where i could upload a file disappears quickly upon page load or page refresh. now I don't know how else to send my files for feedbacks.
After you submit a Support Request on the site you highlight [ https://microsoftedgesupport.microsoft.com/hc/en-us/requests/new ], a way you could add a file to that request is to respond to the confirmation email (that acknowledges that your support request has been received). You would attach your file to the email response.