Customizer Upload Image / Use as Custom Header

gistfile1.phtml

          $wp_customize->add_section( 'header_background', array(
                    'title'             => __( 'Custom Header Image', 'onehundred' ), 
                    'priority'          => 40,
          ) );

          $wp_customize->add_setting( 'header_background', array(
                    'sanitize_callback' => 'construction_sanitize_file_url',
          ) );

          $wp_customize->add_control( new WP_Customize_Image_Control( $wp_customize, 'header_background', array(
		'section'           => 'header_background',
		'settings'          => 'header_background',
                    'label'             => __( 'Select an Image', 'onehundred' ), 
	) ) );
	
	
function construction_sanitize_file_url( $input ) {
	$url = '';

	$filetype = wp_check_filetype( $input );

	if ( $filetype["ext"] ) {
		$url = esc_url_raw( $input );
	}

	return $url;
}


// Add Custom Header
function onehundred_theme_header_css() {
	$header_css = get_theme_mod( 'header_background', '');

	if ( ! empty( $header_css ) ) { ?>
		<style type="text/css" id="header-css">
                              .site-header { background-image: url('<?php echo esc_url( $header_css ); ?>') !important }
                    </style>
	<?php }
}
add_action( 'wp_head', 'onehundred_theme_header_css' );