Downloading attachment from a REST POST response with content-type multipart/mixed


My goal is to download an attachment from a REST POSAT response. The response header content-type value is multipart/mixed and a boundary.
I used multipart reader to extract the file. With the help of regex I have set the Content-Type property of multipart reader as “multipart/mixed;boundary=_Part_16478_1527467874.1613153938198”.
I try to write file afterwards. But multipart reader doesn’t produces any output (file writer is never executed).

Your suggestions will be helpful in finding me the right solution.