Created Unassigned: LESS not being transformed [103]

Hi, I've installed BundleTransformer.Less and JavaScriptEngineSwitcher.Msie. My .less file is being included correctly, and it's being minified correctly (in release mode), but the LESS to CSS transform is not being applied. My .less file contains a single @import directive. That file is also being served directly as a download, rather than displayed in the browser like a .css file would be.

I have a previous project using an earlier version of BundleTransformer which works fine, but I can't spot any significant differences if I compare them.

BundleConfig.cs and web.config are attached. Can you tell me what I've done wrong, please? Thanks!