How to check if a result is a document?

I am using JavaScript that makes a call to a server (Java) to get documents. It has an Ajax call:

Client (JavaScript):

$.ajax({
    type: "get",
    url: urlStringCustomerInvoices,
    contentType: "application/json; charset=utf-8",
    dataType: "text",
    success: function (result) {
        if (result.length > 0) {

The server (Java) sets a byte array representing the PDF document (can also be other document types):

OutputStream os = response.getOutputStream();
bos = new BufferedOutputStream(os);
bos.write(bytes);
bos.flush();
os.flush();
os.close();

This works as expected, i.e. a document, e.g. PDF can be downloaded sucessfully. However, when there is a problem, I want to be able handle it. If the returned result is not a valid document.

On the client in JavaScript, how do I validate if the returned result is a valid document?

Valid documents are file types .pdf, .doc and .docx

The type seems to always be String:

console.log(Object.prototype.toString.call(result));

prints:

[object String]

Also:

console.log(result.length);

prints (for a valid PDF):

66402