This is a 32bit binary, which I think needs some pretty old kernel version. Hence it only works on 4.0, I will try and get an updated release for 4,1 (*note) ESXi 5 comes with sftp-server already.
I came across something interesting while fiddling earlier, after spending about 2 hours building a static release of openssh server that was going to replace dropbear. I’d gotten to a point where I could build a i386 release of the binaries with no random library requirements and sshd would start and listen on a port defined in /etc/ssh/sshd_config. unfortunately starting ssh in debug mode allowed me to see numerous glibc errors during connections and explain why I couldn’t connect. At this point I don’t think there is any real way of replacing dropbear with a complete openssh solution even statically linking. Even testing the openssh sftp binary that had been compiled showed that it wasn’t coping with a system call not returning UIDs correctly meaning that it would report a FATAL error and close continually.
Given openssh wasn’t going to be replaced I researched about dropbear and if there was a newer version perhaps with sftp, unfortunately not. Eventually I came across notes on a blog mentioning that dropbear “supports” openssh sftp. After restoring ESXi back to its default filesystem settings (ssh enabled) it appears the attempting to sftp to esxi returns the following error.
After compiling a slightly older version of openssh (static) I found a release of sftp-server that will once placed in /sbin on ESXi allows full usage of sftp (including sshfs mounting) binary below.