function _arrayBufferToBase64( buffer ) {
    var binary = '';
    var bytes = new Uint8Array( buffer );
    var len = bytes.byteLength;
    for (var i = 0; i < len; i++) {
        binary += String.fromCharCode( bytes[ i ] );
    }
    return window.btoa( binary );
}

ajax:
$.ajax({
    url: "@Url.Action("SaveImage")",
    type: "POST",
    data: formdata,
    processData: false,
    contentType: false
})

Controller:
[HttpPost]
public void SaveImage(string base64string)
{
....
}