I have sucessfully transfered files between 2 clients running Spark 2.6.0RC1 on Windows 7, so I know that is possible.
Trying to transfer files between 2 clients running 2.6.0RC1 on Windows 7 and Mac OS X Snow Leopard has been unsuccessful.
On the Windows side, I can initiate a file transfer request, but the Mac user never even sees that a request pending (this is not a case of the actual transfer having difficulty: if the request never makes it the transfer can’t even try to begin).
On the Mac side, I do not see any way to initiate a file transfer (see http://community.igniterealtime.org/thread/43452)