Although you may find a copy of mail.jar and activation.jar under $NX_ROOT\java\lib, neither the JavaMail package nor the named files are mentioned in the Supportability Matrix and so they are unsupported.
However, it is possible that you could successfully use the files with your implementation. The files can be copied to the $NX_ROOT\CA\SC\tomcat\7.0.23\lib folder for that purpose.
JavaMail is an open source project. For JavaMail 1.5.6, mail.jar is not included; instead, javax.mail.jar is included.
You may download JavaMail 1.5.6 (or possibly a later version of JavaMail) from the primary official source web page - please see the Additional Information section for a suggested link.
From within the downloaded package, you could copy the file named "javax.mail.jar" into the $NX_ROOT\CA\SC\tomcat\7.0.23\lib directory. It should then be possible to extend the use of attaching files to CA SDM tickets via the intended method.