Allow for 'renaming' for file when attaching to entity

Optional parameter for uploadFile method, dstFilename, which allows the user to 'rename' a file when attaching it. Useful for automated systems that have result files stored with generated temporary files names.

Also fixed bug where user provided sessionToken wasn't stored in Synapse.sessionToken (it was stored in Synapse.headers["sessionToken"], but it needs to be stored both places for all the code to work