Hello @sdx111,
You can flash the file with the sdcard suffix with no problem or rename it yourself with no issues. If you require an multicore application I can recommend checking AN13750: Enabling Multicore Application.
If you keep having problems please share more details about the complete process you are following, this is: Bootloader configuration, Changes to ATF/u-boot code, building commands, flashing commands and dip switch configuration when booting.
Let me know if this information was useful.