Files displayed as links have not working url.

First, try this with the current version of Mime Mail.
Second, if that doesn't help, try this without involving Webform. I suggest using the mimemail_example module that comes with Mime Mail.
Third, please provide specific steps to reproduce this without using a complicated module like Webform. A copy of the raw HTML of the email received (including headers) plus a copy of raw HTML passed to Mime Mail would help.

Thanks for your answer.
The alpha 5 was released a few days after the problem appeared and has the same version name that the alpha4 so I get fooled.
Good for me it fixes the issue.

Automatically closed - issue fixed for 2 weeks with no activity.