A repeat on an earlier issue, changing to

$fn = DataObject::get_by_id("MonkeysFist", 7);
$filepath = "assets/Uploads/mf_attachments/$fn->MFFileName";
$emailToSubmiter->attachFile($filepath, $fn->MFFileName);

Fixes it

Bizarre stuff, indeed. If any other attachments get cheeky you may want to get a little more hands on and ensure the encoding, contents, and mime type are specified. With certain finicky MUAs out there, sometimes you must apply some thorough beatings until they yield.

An example from the codes:

    $email = new Email();
    $email->attachments[] = array(
      'filename' => BASE_PATH . "/themes/mytheme/images/header.gif",
      'contents' => file_get_contents(BASE_PATH . "/themes/mytheme/images/header.gif"),
      'mimetype' => 'image/gif',
      'contentLocation' => Director::absoluteBaseURL() . "/themes/mytheme/images/header.gif"
    );