Image extension issue in mime.php

All,

  I was looking at an HTML email today that had an image URI that was
an .asp file.  SM blocked it, even when I clicked to view unsafe
images.... and that's because of the .asp file extension.  SM replaces
all images in HTML view with a blank image unless they are simple
image files with .jpg, .gif, .jpeg, .xjpeg, .jpe, .bmp, .png, or .xbm
extensions.  In today's world, I think there are probably a lot of
images being served dynamically, with URIs that have PHP, JSP, ASP or
some other file extension.  So, in a lot of cases, these should be
allowed and are not necessarily threatening or ill-intentioned.

  Can someone explain the rationale of keeping the list more
restricted?  What can a malicious image URI do if we open the list up
to such file extensions?  Really, if an attacker wanted to do
something here, they could easily circumvent this restriction by
putting a URI with a "valid" (say .png) extension that was really a
php file that is dynamically executed on the target server.  So what
does SM *GAIN* by keeping this list of known image extensions?  (What
we *LOSE* is proper display of many valid HTML mails for our users.)

  My feeling is that this should be addressed by either removing the
restriction list completely, adding .asp, .php, .jsp, and any other
common types, or putting a new configuration value in the config file
for admins who would like to do this themselves.

Thoughts please?

 - Paul

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