Ok, after looking at this again it was the other way around. If your ISO is extracting with the swdl.iso filename, you should be good to go.
I did come across something else that could explain the issue, i pulled this from another fourm:
"The guys at customtronix who make the RAX jailbreak have gotten the 15 software to work on 14 radios but apparently the sd card reader/usb port/jack package in the cubby is different and needs a different driver or something so it's not plug and play."
Is it possible that the 15 radio and your 14 usb reader have a driver compatibility issue? Does your USB stick have a light on it indicating it is receiving power?
On my first RA3, I could begin the update process but it would fail upon restart because the USB reader was no longer powered properly (it would turn on and then off repeatedly). I tired to rectify this by using an external hd that I powered externally, but it still failed to update after the initial reboot. I solved the problem by ordering this from Amazon:
http://www.amazon.com/gp/product/B00JK1OFOM/ref=oh_aui_search_detailpage?ie=UTF8&psc=1
I used this adapter to bypass the USB reader completely and plugged directly into the USB port on the back of the 8.4A (where the grey plug goes). The update worked as long as the USB flash drive was plugged directly into the radio.
The 8.4AN RA4 I recently received was from a 2014 and I did not have to use the adapter to flash which leads me to believe my initial problem may have been driver related.
It's $6.99 to give it a try...