Total concerns:
$data = 'data:image/png;base64,AAAFBfj42Pj4';
// Extract base64 file for standard data
$fileBin = file_get_contents($data);
$mimeType = mime_content_type($data);
// Check allowed mime type
if ('image/png'==$mimeType) {
file_put_contents('name.png', $fileBin);
}