0kb attachments

Where can we see the website in question?

https://vacatures-isbgroep.nl/

On which page do you have the form having the issue?

All pages containing a contact7 form.
For example: https://vacatures-isbgroep.nl/vacature/customer-service-employee-2/

See Setting up mail

What email settings do you have in the Mail tab panel?

It is set up correctly but still about 1 out of 10 mails arrives with a 0kb attachment.

And if we contact the user to mail the file with Outlook, it does arrive correctly, so the file does work.