//only these file extentions are allowed
var re = /^(\.pdf)|(\.dot)|(\.doc)|(\.xls)|(\.txt)/; 

function postIt() {
  	var file = document.getElementById('mainForm').upload.value;
	 	if(file.match(re)) {
	 		document.forms[0].submit();
			return true;
	  	}else if(file!='') {
	  		alert('Please select a valid PDF, DOT, DOC, XLS or TXT file.');
	  		return false;
	  	}
	  	return true;
}