Why the attachment being blocked?

This is using MailScanner on CentOs. There is an external party tried to send us, and in return, mail being bounced back with the error messages as follows:

This is a message from the MailScanner E-Mail Virus Protection Service
----------------------------------------------------------------------
The original e-mail attachment "%112233aaccbb.htm"
is on the list of unacceptable attachments for this site and has been
replaced by this warning message.

If you wish to receive a copy of the original attachment, please
e-mail helpdesk and include the whole of this message
in your request. Alternatively, you can call them, with
the contents of this message to hand when you call.

At Thu Mar  3 22:56:39 2011 the virus scanner said:
   MailScanner: Filename trying to hide its real type (%aabbcc1122.htm)

Note to Help Desk: Look on the abc networks (smtp.abc.com) MailScanner in /var/spool/MailScanner/quarantine/20110303 (message p23EuYdx006926).
--
Postmaster
abc networks

What's wrong with the attachment? How to solve the problem?