Path Format Not Supported.

Question

I Write Code For Download All as zip.

-----------------

string filepath = Server.MapPath("download/flag.png");

zip.AddFile(filepath, folder);

Response.Clear();

Response.AddHeader("Content-Disposition", "attachment; filename=DownloadedFile.zip");

Response.ContentType = "application/zip";

zip.Save(Response.OutputStream);

Response.End();

-----------------

its Completely Working
But When i give DropBox Public path as filepath
--------------------------
string filepath = "https://dl.dropboxusercontent.com/u/155062858/flag.png";
--------------------------

its Occur Error The given path's format is not supported.

Thanks in Advance.