In https://jazz.net/wiki/bin/view/Main/ResourceOrientedWorkItemAPIv2,
Link collection named as "rtc_cm:com.ibm.team.workitem.linktype.attachment.attachment" defined.

and "Adding Links to a Work Item" section says that adding a link to a workitem is possible.

Adding a attachement would be 2 phase.
1) upload a file and create a object(called "attachement) to represent the uploaded file
2) make a link from a workitem to a object("attachement")

Is it possible by OSLC?