Fix shadowed variables in api4 package (#10010)

api4/file.go

@@ -317,7 +317,7 @@ NEXT_PART:
 		filename := part.FileName()
 		if filename == "" {
 			var b bytes.Buffer
-			_, err := io.CopyN(&b, part, maxMultipartFormDataBytes)
+			_, err = io.CopyN(&b, part, maxMultipartFormDataBytes)
 			if err != nil && err != io.EOF {
 				c.Err = model.NewAppError("uploadFileMultipart",
 					"api.file.upload_file.read_form_value.app_error",