The HTTP If-Range request header makes a range request conditional.
If the condition is fulfilled, a range request is issued, and the server sends back a 206 Partial Content response with part (or parts) of the resource in the body.
If the condition is not fulfilled, the full resource is sent back with a 200 OK status.... A weak entity tag (one prefixed by W/ ) must not be...used in this header. <day-name> One of Mon , Tue , Wed , Thu , Fri...