code: static public Integer getAttachmentPosition(
  			JSONObject targetDoc
  			,String attachmentName
  			) {
  		
  		JSONObject targetAttachments = targetDoc.optJSONObject("_attachments");
  		if( null == targetAttachments ) {
  			// No attachment on target doc
  			return null;
  		}
  		
  		JSONObject targetAttachment = targetAttachments.optJSONObject(attachmentName);
  		if( null == targetAttachment ) {
  			// Target document does not have an attachment with this name
  			return null;
  		}
  		
  		if( JSONSupport.containsKey(targetAttachment,"revpos") ){
  			int revPos = targetAttachment.optInt("revpos",-1);
  			if( revPos < 0 ){
  				return null;
  			}
  			return revPos;
  		}
  		
  		return null;
  	}
docs: Given a JSON document and an attachment name, returns the
  revision position associated with the attachment.
  @param targetDoc
  @param attachmentName
  @return