You don't need it, you only need to see the file name, when it tries to do it, so you can name it accordingly.
And it's already there, in the thread you said you've read Adding OpenWrt support for Xiaomi AX3600 - #3691 by ajw107
But uboot access would require writable console, which I assume you don't have, so automagic tftp is your only easy way forward.