fairly a good question. i can only assume that it does not make a difference that much because it starts with text/, but i can't tell you, because this is very specific about which robot is looking for it etc. any can do such a check for mimetype or not. maybe someone here made a testscenario about that already, but i don't know.
if you want to have this corrected (on my server it's text/plain and i have no probs), contact your server admin.