XML based email attachment being renamed

Question

I am experiencing a strange problem when receiving email attachments.

The files are valid XML files, but they have the file extension .usf or.xif.

When the emails are opened in Outlook and the attachments are saved, the file extension is changed to .xml?

I am using fastmail.fm as my server.

The senders are using webmail or Thunderbird.

Is this a problem with the sender, fastmail or outlook?

Answer 1

My guess is that it's Fastmail. I tested Thunderbird and Outlook using the scenario you outlined, and the file extension was not changed.

It seems that some vulnerabilities were exposed in Fastmail earlier this year (this and this), and it's possible that .XIF and .UFS have been identified as executable file formats, or dangerous for other reasons.