It should be text/css... have you used dot in the extension?
Quoting a "solution" marked stackoverflow.com post:
The extension is .less and the MIME Type is text/css (by – mattruma Nov 7 '11 at 21:42)
...
another post suggests:
"When using Asp.Net you can add the mime type in your web.config:"
<system.webServer>
<staticContent>
<mimeMap fileExtension=".less" mimeType="text/css" />
</staticContent >
</system.webServer>
You can also try to exclude the path with .less in the CMS (URL settings).
Regards,
Zdenek