var dataForm = new FormData();

dataForm.append("metadata", JSON.stringify({
   "fileName": attachment.files[0].name,
}));

dataForm.append("file", attachment.files[0]);

$.ajax({
   url: "http://mipagina.openproject.com/api/v3/work_packages/{ID}/attachments",
   method: "POST",
   enctype: 'multipart/form-data',
   processData: false,
   data: dataForm,
   cache: false,
   success: function (data) {
      console.log(dataForm);
      console.log(data);
   },
   error: function () {
      console.log("error");
   }
 });