xmlhttprequest-overridemimetype-allowedState.html

<!DOCTYPE html>
<script src="/js-test-resources/js-test.js"></script>
<script>
jsTestIsAsync = true;

function testOverrideMimeType(xhr) {
    debug("Testing overrideMimeType when readyState is " + xhr.readyState);

    var isAllowed = xhr.readyState < XMLHttpRequest.LOADING;
    if (isAllowed) {
        shouldNotThrow("xhr.overrideMimeType('text/plain')");
    } else {
        shouldThrow("xhr.overrideMimeType('text/plain')");
    }
}

var xhr = new XMLHttpRequest();
xhr.onreadystatechange = function() {
    testOverrideMimeType(xhr);

    if (xhr.readyState == XMLHttpRequest.DONE)
        finishJSTest();
}

testOverrideMimeType(xhr);
xhr.open("GET", "resources/get.txt", true);
testOverrideMimeType(xhr);
xhr.send(null);
testOverrideMimeType(xhr);
</script>

chromium/third_party/blink/web_tests/http/tests/xmlhttprequest/xmlhttprequest-overridemimetype-allowedState.html