The "File Name" file filtering rule is configured to block the term *.eml
In Exchange 2007 the content-type of attached .eml files is "message/rfc822". When Symantec Mail Security for Microsoft Exchange (SMSMSE) replaces this file according to file filtering rule, it changes the name of file to "Deleted Attachment.txt" but it doesn't change its type to"text/plain" from "message/rfc822".
This problem is fixed in Symantec Mail Security 6.0.10 for Microsoft Exchange and later, including version 6.5. To learn how to obtain the latest version, read Where to download the latest version of Symantec Mail Security for Microsoft Exchange.