create the final file with mode 0666 for multipart-uploads (#9173)

NAS gateway creates non-multipart-uploads with mode 0666.
But multipart-uploads are created with a differing mode of 0644.

Both modes should be equal! Else it leads to files with different
permissions based on its file-size. This patch solves that by
using 0666 for both cases.

pkg/ioutil/append-file_nix.go

@@ -25,7 +25,7 @@ import (
 
 // AppendFile - appends the file "src" to the file "dst"
 func AppendFile(dst string, src string) error {
-	appendFile, err := os.OpenFile(dst, os.O_WRONLY|os.O_APPEND|os.O_CREATE, 0644)
+	appendFile, err := os.OpenFile(dst, os.O_WRONLY|os.O_APPEND|os.O_CREATE, 0666)
 	if err != nil {
 		return err
 	}