document.getElementById("upload_button").onchange = function () {
document.getElementById("upload_file").value = this.value;
};

<div id="Form_FileInput_Container"><input type="file" id="upload_button" /></div>
<input class="Form_Input" id="upload_file" placeholder="Choose File" disabled="disabled" />

#Form_FileInput_Container { position: relative; width: 100px; height: 100px; background: #e5e5e5 url('path/to/upload/image/forms_upload.png') no-repeat; border: 1px solid #cccccc; }
#Form_FileInput_Container input { filter: alpha(opacity=0); opacity: 0; width: 100px; height: 100px; }

<input type="file" id="upload_button" onchange="cutomButton()" />

function customButton() {
    var z=document.getElementById("upload_button").value;
    document.getElementById("upload_file").value = z;
    alert(document.getElementById("upload_file").value);
}