Which status code should I return?

Question

I am doing an dropbox.js upload. I am checking for errors on the server and returning them to my frontend via json. So far so good.

 header('HTTP/1.1 500 Internal Server Error');
            header('Content-Type: application/json; charset=UTF-8');
            die(json_encode(array('message' => 'ERROR', 'code' => 100)));

on my frontend I can catch the error and do some nice stuff to alert the user

error: function(file, errorMessage,  xhr) {
     console.log(xhr);
     // catch the error and alert the user
  }

The problem is the errors are not really 'server errors' so how should I return them in the header? A 500 does not seem appropriate. It does not I suppose really matter but would nice to return the right type of error.

The kind of errors include

• filename is not unique
• wrong file type
• file is too large

What is the best status code to use?

Thanks.