Suggests that the image you are uploading is not the right format.
From the last time I needed to recover my Archer C7v2 by TFTP
[jeff@miniup ~]$ ls -l /private/tftpboot/
total 59820
lrwxr-xr-x 1 root wheel 61 Feb 21 2018 ArcherC7v2_tp_recovery.bin -> lede-17.01.4-ar71xx-generic-archer-c7-v2-squashfs-factory.bin
[...]
Note that this is an old image and a current -squashfs-factory.bin should work as well.