If you use Eclipse client, you are able to add attachment to Notes of Plan but Web client does not have the same feature for now. Please find the info in step 10:

http://www-01.ibm.com/support/knowledgecenter/SSYMRC_5.0.0/com.ibm.team.apt.doc/topics/t_create_overview.html?lang=en

There is a enhancement story workitem to request the same feature in web client and you may want to comment on it:
https://jazz.net/jazz/web/projects/Rational%20Team%20Concert#action=com.ibm.team.workitem.viewWorkItem&id=296466