Fix bug: saving in a plain CSS inline editor generates copy of CSS file

Saving in a CSS inline editor was running `compileLess()` even though it's not a LESS file (generating a "compiled" copy of the CSS file with one letter missing from its filename).

This bug could also have other symptoms in other cases, e.g. saving in a JS inline editor or saving many files at once via Save All could try to "compile" files of all sorts of other types too.