robots.txt error : Content Type should be text/plain

I'm testing my site with the software called Search Engine Optimization (SEO) Toolkit 1.0 it displays this error :

The content type for the response from "htpp://mysite.com/robots.txt" is "text/html". The Web server should return "text/plain" as the content type for a Robots.txt file.

My robots.txt file is simply this :

User-agent: * 
Allow: /

Saved with UTF-8 Without BOM Encoding.

Is this wrong?

What should be a default, harmless robots.txt file?

Thanks !