Determines if a custom logo SVG format when uploaded to WordPress Customiser

custom-logo.php

/** Add SVG support first **/
function wcd_add_svg_support($file_types){
    $new_filetypes = array();
    $new_filetypes['svg'] = 'image/svg+xml';
    $file_types = array_merge($file_types, $new_filetypes );
    return $file_types;
    }
add_action('upload_mimes', 'wcd_add_svg_support');

/*
 * Determine logo and declare output
 * to use in theme you just need to us the following code
 *  <?php wcd_logo(); ?>
 */
function  wcd_logo()
{
    /** Get our custom logo */
    $custom_logo_id = get_theme_mod('custom_logo');
    /** Get custom logo full url */
    $logo = wp_get_attachment_image_src($custom_logo_id, 'full');
    /** Get the extension of our logo ie: svg, jpg, png etc */
    $extension = end(explode('.', $logo[0]));

    /** first check if there is a logo uploaded */
    if (has_custom_logo()) {
        /** now check if it is an svg file */
        if ( $extension != 'svg' ) {
            /** if it is not svg, just output the image with homepage link */
            $custom_logo = '<a href="' . get_site_url() . '" class="logo navbar-brand"><img src="' . esc_url($logo[0]) . '"></a>';
        } else {
            /** it is svg so lets get the svg code from the uploaded file */
            $svg_file = file_get_contents($logo[0]);

            /** output the svg inline for custom styling with homepage link */
            $custom_logo = '<a href="' . get_site_url() . '" class="logo navbar-brand">' . $svg_file . '</a>';
        }
    } else {
        /** there is no logo, so lets output the site title */
        $custom_logo = '<a href="' . get_site_url() . '" class="navbar-brand">' . get_bloginfo('name') . '</a>';
    }
    echo $custom_logo;
}