Let administrators upload SVG images

<?php

// Only users who may post unfiltered HTML (administrators). Upload only SVG
// files you made or trust.
add_filter(
	'upload_mimes',
	static function ( $mimes ) {
		if ( current_user_can( 'unfiltered_html' ) ) {
			$mimes['svg'] = 'image/svg+xml';
		}
		return $mimes;
	}
);

// PHP may report an SVG as plain text; tell WordPress what it really is.
add_filter(
	'wp_check_filetype_and_ext',
	static function ( $data, $file, $filename ) {
		if ( ! current_user_can( 'unfiltered_html' ) || 'svg' !== strtolower( pathinfo( (string) $filename, PATHINFO_EXTENSION ) ) ) {
			return $data;
		}
		$start = is_readable( $file ) ? (string) file_get_contents( $file, false, null, 0, 4096 ) : '';
		if ( false !== stripos( $start, '<svg' ) ) {
			$data['ext']  = 'svg';
			$data['type'] = 'image/svg+xml';
		}
		return $data;
	},
	10,
	3
);