No attachment is sent

I have installed the latest (6.x-1.0-rc4) Simplenews module along with the latest (6.x-1.x-dev) Mime Mail module available for Drupal 6.9.

When adding a newsletter issue, I can upload files as attachments, and they appear in the previewed/saved node.

However, there is *no attachment in the e-mails* sent by Simplenews.

I have tried it both with plain text and HTML.
(I can send e-mails in HTML, which I believe inidicates that the Mime Mail module works.)

There is very little about attachments in the module's documentation.

If anyone ever managed to get attachments work in Drupal 6.x, their help would be highly appreciated.