Allow only pdf, doc, docx format for file upload?

Question

I am triggering a file upload on href click.
I am trying to block all extension except doc, docx and pdf.
I am not getting the correct alert value.

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Answer 1

var file = form.getForm().findField("file").getValue();
var fileLen = file.length;
var lastValue = file.substring(fileLen - 3, fileLen);
if (lastValue == 'doc') {//check same for other file format}