Don't see how the MIME type would do anything for this issue.
You can avoid caching by appending a random number to the url (this is a general 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)
);
More info here
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