How to check file MIME type with javascript before upload?

Question

I have read this and this questions which seems to suggest that the file MIME type could be checked using javascript on client side. Now, I understand that the real validati

Answer 1

If you just want to check if the file uploaded is an image you can just try to load it into tag an check for any error callback.

Example:

var input = document.getElementsByTagName('input')[0];
var reader = new FileReader();

reader.onload = function (e) {
    imageExists(e.target.result, function(exists){
        if (exists) {

            // Do something with the image file.. 

        } else {

            // different file format

        }
    });
};

reader.readAsDataURL(input.files[0]);


function imageExists(url, callback) {
    var img = new Image();
    img.onload = function() { callback(true); };
    img.onerror = function() { callback(false); };
    img.src = url;
}