title: How can I get the file's total size without content-length
body: <p>I want to download some files and show the downloading progress with progressview. The format is currentSize/totalSize. As you know, I can get the total size using content-length of http response. However, sometimes you cannot find the attribute in headers.
    So, my question is how can I get the total size of file to be download? Or is there any way to make content-length reappear in headers.</p>