There are two main reasons for this step. First of all I am not able to open these files and I believe I will not be able to do so and get them properly rendered anytime soon. Secondly, people using the new Microsoft Office suite seem to be ignorant enough to think everyone is able to view those files, which is not the case.
I am trying to make one point here:
People sending emails to other people should always send files in internationally standardized formats (open formats), such as ODF or PDF, so that everyone is able to open them and use the attachments. Also, I am trying to make people sending out emails in those formats aware of the fact that not everyone can open them, not everyone wants to invest a lot of money in new applications and that some people generally prefer Free Software and that there is no way of using those files using Free Software right now.
Enough for the introduction, I wanted to explain how to achieve this behavior using Exim4:
deny message = Message contains attachment of unwanted type ($found_extension)
demime = docx:pptx:xlsx
Putting this snippet in the acl_check_content section of your exim4.conf should do the trick.
Oh, and while I am at it, you can easily use this snippet to drop mails with other attachments, based on the file extension.
For example, in order to reject all mails containing WMV files just use demime = wmv.
Note that this snippets checks for a specified file extension instead of a MIME type. People still can get mails through in those formats if they modify the file extension, so do not use this method as a security measure.