mail-addons/mail_attachment_popup/static/src/css/styles.css

.g-hidden {
    display: none;
}
.box-modal img {
    max-width: 900px;
    width: 100%;
}
.box-modal-li li {
    list-style-type: none;
}
.box-modal-li {
    padding-left: 0;
}
.o_attachment .o_image {
    cursor: pointer;
}