Symfony version(s) affected: 5.3.6

Description
Mailgun transport silently ignore any attachments with non-ascii characters in its name like Confirmación cliente. If I remove ó from filename, it works as expected.
Not that with smtp transport, it work fine.

NOTE: It used to work fine with swift-mailer-bundle with mailgun transport.

How to reproduce
Here is a reproducer.
https://github.com/faizanakram99/symfony-mailer-mailgun-bug-reproducer

Possible Solution

Additional context
I guess the actual problem might be in symfony/http-client , considering it works with smtp transport