标题css / bootstrap问题，很奇怪

I do not understand what is going on here.

This is my file-data "structure":

enter image description here

Over here is the content of my folder "css"

enter image description here

So, in my program: this files"estudiante.php","editar.php" and "hola.php" got the same header. Take a look:

    <head>
        <meta name="viewport" content="width=device-width, initial-scale=1">
        <script src="http://code.jquery.com/jquery-2.1.1.min.js"></script>
        <meta name="viewport" content="width=device-width, initial-scale=1">
        <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css" integrity="sha384-BVYiiSIFeK1dGmJRAkycuHAHRg32OmUcww7on3RYdg4Va+PmSTsz/K68vbdEjh4u" crossorigin="anonymous">
        <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap-theme.min.css" integrity="sha384-rHyoN1iRsVXV4nD0JutlnGaslCJuC7uwjduW9SVrLvRYooPp2bWYgmgJQIXwl/Sp" crossorigin="anonymous">
        <script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/js/bootstrap.min.js" integrity="sha384-Tc5IQib027qvyjSMfHjOMaLkfuWVxZxUPnCJA7l2mCWNIpG9mGCD8wGNIcPD7Txa" crossorigin="anonymous"></script>


</head>

The problem is that i could not link the file "estilos.css" from the "css" folder because i was given some errors while i was checking the inspect element tool using chrome:

enter image description here

But it is strange, because i wrote this line on my header:

<meta name="viewport" content="width=device-width, initial-scale=1">

That line makes my page responsive, but those commands are in my estilos.css file.. i don get it, take a look:

enter image description here

So, estilos.css doesnt appear in my code but the header uses the content and makes my website responsive?. I dont understand this mates :(

Double-check your file naming conventions. It looks like you're hiding file extensions, since bootstrap, bootstrap.min, etc don't have .css on the end, but estilos.css does. It's possible your file is actually named estilos.css.css and you aren't aware.

https://www.howtogeek.com/205086/beginner-how-to-make-windows-show-file-extensions/