PHP7.4 deprecates first implode parameter being pieces, second glue

lib/swiftmailer_generate_mimes_config.php

@@ -173,7 +173,7 @@ function generateUpToDateMimeArray()
     ksort($valid_mime_types);
 
     // combine mime types and extensions array
-    $output = "$preamble\$swift_mime_types = array(\n    ".implode($valid_mime_types, ",\n    ")."\n);";
+    $output = "$preamble\$swift_mime_types = array(\n    ".implode(",\n    ", $valid_mime_types)."\n);";
 
     // write mime_types.php config file
     @file_put_contents('./mime_types.php', $output);