contentType option to false is used for multipart/form-data forms that pass files.
When one sets the contentType option to false, it forces jQuery not to add a Content-Type header, otherwise, the boundary string will be missing from it. Also, when submitting files via multipart/form-data, one must leave the processData flag set to false, otherwise, jQuery will try to convert your FormData into a string, which will fail.
Use jQuery's .serialize() method which creates a text string in standard URL-encoded notation.
You need to pass un-encoded data when using contentType: false.
Try using new FormData instead of .serialize():
var formData = new FormData($(this)[0]);
See for yourself the difference of how your formData is passed to your php page by using console.log().
var formData = new FormData($(this)[0]);
console.log(formData);
var formDataSerialized = $(this).serialize();
console.log(formDataSerialized);