For us, I found it to be a bug in Safari not checking the required attribute for this field.  My solution was to add a checkbox below for the user to verify if a file was attached.  When they click it I check the upload field, VerifyFileAttached, for how many files are attached.  If it is how many files I anticipate, I leave the box checked and display the submit button.  If the number of files isn't what I'm looking for then I uncheck the box and hide the submit button.  See code below:

      $('.VerifyFileAttached input').change(function(){
        var fileAttch = document.querySelectorAll('.file').length;
          if ($(this).prop('checked') && fileAttch == 1){
          $('.Submit').show();
          }
          else{
          if (fileAttch == 0){
            alert("Please upload a file to continue");
          }
          $('.Submit').hide();
          $('#Field81-0').prop('checked', false);
          }
      });