Actually... following keef's moaning.....
zsh
autoload -U zmv
cd /Users/mixture/Desktop/folder
noglob zmv -W *.exe *.jpg
or in a couple of lines.....
cd /Users/mixture/Desktop/folder
zsh -c "autoload zmv; noglob zmv -W *.exe *.jpg"
Would probably be the visually cleanest way to do it on a mac (omitting the fat-finger protection).
For keef's benefit, you could probably even go one step further.....
zsh
autoload -U zmv
alias rename='noglob zmv -W'
then you just go....
cd /Users/mixture/Desktop/folder
rename *.exe *.jpg