<form enctype="multipart/form-data" id="modal_form_id" method="POST" >
<input type="file" name="documents">
</form>
var postData = new FormData($("#modal_form_id")[0]);
$.ajax({
type:'POST',
url:'your-post-url',
processData: false,
contentType: false,
data : postData,
success:function(data){
console.log("File Uploaded");
}
});