Getting an IAttribute object from an attribute id on a server extension

You probably need to use IWorkItemServer (which extend IWorkItemCommon) as prerequisite and in the getService method. Then you can also use it for the rest of the operation or cast it to IWorkItemCommon.

https://rsjazz.wordpress.com/2012/07/31/rtc-update-parent-duration-estimation-and-effort-participant/ shows how this is done in a participant/follow up action and the same mechanism applies in advisors/precondition. It is basically the same mechanism explained in https://jazz.net/forum/questions/226930/how-to-get-icontributor-objects-from-a-workitem where you don't specify the context.

You should use IWorkItemCommon wherever you can and it is possible to use it here. I don't know why you can't get it. I agree with Michele on the alternative and the example above uses IWorkItemServer an works. 

 I had to recreate the plugin project for the WorkItemServer to work. weird...