How can I get zsh's completion working in the middle of the filename?

As per the comments, I tried disabling oh-my-zsh, which fixed this problem. I then went through the oh-my-zsh source, selectively disabling modules.

I previously had CASE_SENSITIVE="true", but commenting out this line fixed it for me. Apparently it's a known bug.

To fix it, I could put the following line in ~/.zshrc after sourcing oh-my-zsh.

zstyle ':completion:*' matcher-list 'r:|=*' 'l:|=* r:|=*'

Personally, I prefer this solution much more - http://zsh.sourceforge.net/FAQ/zshfaq04.html#l50, It works like completion in tcsh, and completes words, that are before cursor only.

Just use

    bindkey '\CI' expand-or-complete-prefix

instead of

    bindkey '\CI' expand-or-complete

(Which is by default).