How to check upload file header using java script ?

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JScript.NET

I wants check Header of upload file.For example abc.png is correct format but extension change to abc.doc this file restrict to upload.

What I have tried:

var validExtensions = ['doc','pdf','docx','odt','txt','rtf']; //array of validextensions
var fileName = file.name;
var fileNameExt = fileName.substr(fileName.lastIndexOf('.') + 1);
if ($.inArray(fileNameExt, validExtensions) == -1) {
return false;
}
This is only check for Extensions.Please Help me.