LayoutTests/fast/files/blob-content-type.html

<script src="../../resources/testharness.js"></script>
<script src="../../resources/testharnessreport.js"></script>
<script>

    const blob = new Blob(['<style></style>'], { type: 'text/original-blob-content-type;' });

    promise_test(async () => {
        const slice = blob.slice(0, blob.size, 'text/html2;charset=utf-8');
        const url = URL.createObjectURL(slice);
        const response = await fetch(url);
        assert_equals(response.headers.get('content-type'), 'text/html2;charset=utf-8')
    }, 'blob.slice content type used')

    promise_test(async () => {
        const slice = blob.slice(0, blob.size);
        const url = URL.createObjectURL(slice);
        const response = await fetch(url);
        assert_equals(response.headers.get('content-type'), 'text/original-blob-content-type;')
    }, 'sliced-from content type used when none specified in blob.slice')

    promise_test(async () => {
        const slice = blob.slice(0, blob.size, "");
        const url = URL.createObjectURL(slice);
        const response = await fetch(url);
        assert_equals(response.headers.get('content-type'), 'text/original-blob-content-type;')
    }, 'sliced-from content type used when empty string content type specified in blob.slice')

</script>