Use is_file() over file_exists() in MD_STD::ensure_file()

src/MD_STD.php

@@ -704,7 +704,7 @@ final class MD_STD {
      */
     public static function ensure_file(string $filepath, array $accepted_mimetype = []):void {
 
-        if (!\file_exists($filepath)) {
+        if (!\is_file($filepath)) {
             throw new MDFileDoesNotExist("File " . basename($filepath) . " does not exist");
         }