My Editor got an error

My problem is that when I were going to edit my themes. I go to Appearance>>>Editor. Then I clicked Header(header.php) as I want to go for some edit but instead of displaying Header(header.php),it display stylesheet(style.css) . I had tried Comments(comments.php) ,Footer(footer.php) ,etc but it always displaying stylesheet(style.css). Help me to solve this problem.

This kind of problem really make my head blew up anyway.
I hope there is a solution to this problem.