I do not see how the MIME type will do anything for this problem.
You can avoid caching by adding a random number to the url (this is a common practice that has nothing to do with d3). So if your url is
var url = 'http://www.example.com/somthing';
Then make your request
d3.text( url + '?' + Math.floor(Math.random() * 1000) );
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