Just add an exta INNER JOIN like so
Then you can add the field(s) you want to select from CMContFile to the SELECT clause of the query.
MailingsTemp on MailingsTemp.IDNUMBER = CMMaster.IDNUMBER
CMContFile ON CMContFile.IDNUMBER = CMMaster.IDNUMBER
Note that you don't have to specify in which order MySQL needs to JOIN the tables. Stating MailingsTemp first and than CMContFile doesn't mean MySQL will join that way. MySQL will figure out the best way to join the tables for itself*.
* If you really want MySQL to join tables in a particular order --and in 99,9999% of the time there is no reason for this-- you can use the query hint STRAIGHT_JOIN