I recommend forcing it. As far as I'm aware, forcing it will simply just skip the content-type check, but any parse errors will still occur if it can't be parsed.
Ah.. Ok, Then Forcing it sounds alright to me then. I misunderstood the force functionality.
though I can't find this.
I think i've seen something similar, Perhaps part of the RSS2 standard, or one of the XML standards... Here's a drupal ticket about text/plain RSS feeds, some sourses that state which mime types to be used are mentioned. http://drupal.org/node/142895