Have you read https://support.f5.com/csp/article/K01235989 and followed "Disable large file processing by the BIG-IP ASM" ? This was the ASM will still check the first 10KB but will not block big files.
Also do you have security logging profile under the ASM to see that you are blocked by the ASM and do you still see the error messages in /var/log/asm that the file is bigger than the buffer? This "ASM::disable" is not great because you may bypass your security for real attacks also maybe the files are matched with "POST" and you dont trigger the irule? Do you see that the traffic is matching the iRule in the /var/log/ltm?