// Allow SVG
function allow_svg($mimes)
{
    $mimes['svg'] = 'image/svg+xml';
    $mimes['svgz'] = 'image/svg+xml';
    return $mimes;
}
add_filter('upload_mimes', 'allow_svg');

function fix_mime_type_svg($data = null, $file = null, $filename = null, $mimes = null)
{
    $ext = isset($data['ext']) ? $data['ext'] : '';
    if (strlen($ext) < 1) {
        $exploded=explode('.', $filename);
        $ext=strtolower(end($exploded));
    }
    if ($ext==='svg') {
        $data['type']='image/svg+xml' ;
        $data['ext']='svg' ;
    } elseif ($ext==='svgz') {
        $data['type']='image/svg+xml' ;
        $data['ext']='svgz' ;
    }
    return $data;
}
add_filter('wp_check_filetype_and_ext', 'fix_mime_type_svg', 75, 4);

function fix_svg() {
  echo '<style type="text/css">
        .attachment-266x266, .thumbnail img {
             width: 100% !important;
             height: auto !important;
        }
        </style>';
}
add_action( 'admin_head', 'fix_svg' );