iOS has a bug with input[type="file"] where a delay in submitting a form from an HTTPS URL after attaching a file will result in an empty zero byte file being sent to the server. To reproduce the issue, create a form with an input[type="file"] on an HTTPS URL, attach a file, then wait 90 seconds before submitting.
If you develop WordPress sites that use Gravity Forms, the above validation filter present an error message when empty, zero-byte files are uploaded.