jquery-input-file-validate-mime-type.js

    // checks that all files in <input type="file"> have valid mime type (according to 'accept' attribute)
    $.fn.filesHaveValidType = function() {
        // this validation works for:
        //  1. simple 'accept' values like 'image/*'
        //  2. combined 'accept' values like accept="image/png, audio/mp3"
        var elem = this[0];
        var accept = this.attr('accept');
        if (accept) {
            var acceptTypes = accept.split(',').map(function(s) { return s.trim() });
            // iterate over all files and return false if some file is of invalid type
            for (var i = 0; i < elem.files.length; i++) {
                var correctFileType = acceptTypes.some(function(acceptType) {
                    // .match() is used to compare, say, 'image/*' and 'image/png'
                    return elem.files[i].type.match(acceptType);
                });
                if (!correctFileType) {
                    return false;
                }
            }
        }
        return true;
    }