.files_wrapper {
    position: relative;
}
.files_panel {
    display: none;
    position: sticky;
    position: -webkit-sticky;
    bottom: 0;
    margin-bottom: 0;
    width: 100%;
    background-color: #fff;
    padding: 10px 15px;
    border-top: 2px solid #999;
}
.files_panel.active {
    display: block;
}