So I've done this exact thing when a BIOS update failed and left my computer bricked. I remembered having to remove a header from the cap file but I couldn't remember the exact size of the header so I went searching and found the exact post that helped me at the time: https://superuser.com/a/679461
I went back and compared the files I used and it does indeed look like I just had to remove 2kb from the start of the cap file and then I was able to flash it.
It's been 4 years since I've done this so I unfortunately don't remember all the details but hopefully that's somewhat helpful at least!