I ran into the same problem (with a refurb M610, iDRAC firmware was 2.20). I tried to upgrade the iDRAC6 firmware to 3.60 using the iDRAC6 web interface and it was a no go. I tried several browsers (chrome and firefox) and same issue each time.I tried to upgrade to firmware 2.31 (as specified in the release note) instead of 3.60 and same issue each time. I eventually managed to upgrade the iDRAC6 firmware by doing the upgrade from the M1000e web interface and not from the iDRAC6 web interface. It worked first time (using Chrome).
I have blade that is an M610. its current iDRAC6 Firmware version is 3.30 (Build 17), its BIOS version is 6.0.7. I am trying to upgrade the iDRAC firmware upto version 3.50 (Build 4). I am upradging through the CMC on the M1000e chassis. I choose the slot the Blade is in and then Launch the IDRAC GUI. I choose update firmware and select the firming.imc file that I have successfully used on other M610 Blades. After about 30 seconds it comes back with the following error. The uploaded image has failed verification   Why?  
M1000e with M610 Blade. iDRAC6 issues with upgrading firmware to 3.50 (Build 4) 
- "The uploaded image has failed verification" Error