This post adressed a similar issue:
https://www.mobileread.com/forums/sh...d.php?t=349634
But you are talking specifically about getting files from within KOReader I think.
I see now that the stock browser downloads .cbz files fine, so that was not your issue.
So more specifically I would suggest compiling or chrooting the elinks browser, or even easier by copying:
https://www.mobileread.com/forums/sh...91&postcount=3
Maybe even easier is to use your phone to download then relay files by SFTP.
Just run SSH server on KOReader and transfer by SFTP from Ghost Commander+SFTP Plugin(open source but available from Google Play or Aurora) installed on your Android.
**Unfortunately,this was buggy on one of my devices (and needless wear on phone storage for large files.)
What DID work was using the web browser on my phone to copy the html address of the target, then paste this into a ConnectBot (from F-Droid) SSH connection to the Kobo to "wget <address>".
Reliable and easy!