Test that XHR.responseType = 'blob' gives you back a Blob. On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE". PASS xhr.responseType is "blob" PASS successfullyParsed is true TEST COMPLETE PASS xhr.response is null PASS xhr.response is null PASS xhr.response instanceof Blob is true PASS xhr.response.type is "text/plain" PASS xhr.responseType is "blob" PASS xhr.response is null PASS xhr.responseType is "blob" PASS xhr.response is null PASS xhr.response instanceof Blob is true PASS xhr.response.type is "application/octet-stream"