no it wont mess up, not if you use it correctly:
[file.jpg = file.bmp = file.bmp = file.rar = file.zip = file.txt = file.bat = file.exe = file.gif = file.tgz = file.mpg = file.wmv = file.avi = file.mp3 ETC ETC]
<img src="/template/icons/%item-ext%.gif" alt="" />
this way, only the extensions where an icon have been made for will have the %item-ext% feature.
[file.mpeg]
<img src="/template/icons/mpg.gif" alt="" />
Only a few require this, but its worth it.
and last:
[file] -- for every non-set extension
<img src="/template/icons/file.gif" alt="" />