Again. Atm we do not support mass storage. Only MTP. This is due to a huge number of technical issues with mass storage that are not easy to deal with/solve. Even android dropped mass-storage in favour of MTP.

OS X does support rndis with the horndis driver an then you can transfer files over sshfs/scp/sftp

And this was not a question of not supporting OS X, this is rather OS X being broken. Afaik one of the only working mtp transfer programs (android file transfer) also got recently broken by Apple. So in short it is rather Apple making support impossible.

However as stated some kind of mass-storage support might come. Exporting the SD-card or a virtual fs on the device.

Note: The device might sometimes claim to be an EMPTY mass-storage device. This is a trick to make sure it charges correctly if no enumeration happens otherwise. This happens when you select charging only, or do not make a choice when the USB selection menu pops up.