prueba.html

<input type="file"><br>
<div></div>


<script>
    var fileEl = document.querySelector("input");
    fileEl.onchange = function(e) {

        var file = e.target.files[0], // selected file
            mime = file.type, // store mime for later
            rd = new FileReader(); // create a FileReader

        rd.onload = function(e) { // when file has read:

            var blob = new Blob([e.target.result], {
                    type: mime
                }), // create a blob of buffer
                url = (URL || webkitURL).createObjectURL(blob), // create o-URL of blob
                video = document.createElement("video"); // create video element
            console.log(blob);

            video.preload = "metadata"; // preload setting
            video.addEventListener("loadedmetadata", function() { // when enough data loads
                document.querySelector("div")
                    .innerHTML = "Duration: " + video.duration / 60 + "min"; // show duration
                (URL || webkitURL).revokeObjectURL(url); // clean up

                // ... continue from here ...

            });
            video.src = url; // start video load
        };
        rd.readAsArrayBuffer(file); // read file object
    };
</script>