title: Initiating Multipart upload to amazon s3 expecting x-amz-content-sha256 header
body: <p>I am experimenting with libcurl for a multipart upload to s3. My initiate multipart request looks like this</p>
  
  <blockquote>
    <p>POST /my_new_file.mbi?uploads HTTP/1.1   Accept: <em>/</em>  Host:
    test_bucket.s3.amazonaws.com   Date: Thu, 01 May 2014 13:35:17 GMT 
    Authorization: AWS4-HMAC-SHA256
    Credential=XXXXXXX/20140501/us-east-1/s3/aws4_request,SignedHeaders=host,Signature=1a3fd6195040494dd95507455a3b1eefef40346485e3fdafbe6cc136192365a2</p>
  </blockquote>
  
  <p>I get the following response</p>
  
  <blockquote>
    <p>The provided 'x-amz-content-sha256' header must be a valid SHA256.</p>
  </blockquote>
  
  <p>s3 documentation says we do not need any other headers for Initiate multipart upload call(POST). Have tried with various combinations of signed empty content, but no luck. </p>
  
  <p>What am I missing here? Any suggestions here will be very helpful.</p>
  
  <p>Thanks</p>