[Fix] Release attachment should use DownloadURL() not Name (#9529)

* Handle if two or more attachments have the same name
* previously only the first could be downloaded - now each is downloadable
* dos also take care of  #6506 (fix was: #6512)
* use func DownloadURL() at issue attatchments too

routers/repo/repo.go

@@ -426,7 +426,7 @@ func RedirectDownload(ctx *context.Context) {
 			return
 		}
 		if att != nil {
-			ctx.Redirect(setting.AppSubURL + "/attachments/" + att.UUID)
+			ctx.Redirect(att.DownloadURL())
 			return
 		}
 	}