I have a file called "c-faq.txt" containing a
saved Usenet posting of the comp.lang.c FAQ. When
I try to open this file, I get the following warning:

<quote>
Cannot open c-faq.txt

The filename "c-faq.txt" indicates that this file
is of type "Plain text document". The contents of
the file indicate that the file is of type "Email
message/mailbox". If you open this file, the file
might present a security risk to your system.

Do not open the file unless you created the file
yourself, or received the file from a trusted
source. To open the file, rename the file to the
correct extension for "Email message/mailbox",
then open the file normally. Alternatively, use
the Open With menu to choose a specific
application for the file. 
</quote>

This doesn't give any hint to what Nautilus
considers the correct extension for an email message.

(I'll file seperate bugs about Nautilus
misidentifying a Usenet post as a email message,
and there is nothing dangerous about opening
something as a plain text file.)