upstream-mattermost/server/templates/partials/message_attachment.css

.messageAttachments * {
    font-family: sans-serif !important;
}

.messageAttachmentContent {
    padding: 0px;
    border: 1px solid rgba(63, 67, 80, 0.16);
    margin-bottom: 20px;
    border-radius: 0 4px 4px 0;
}

.messageAttachmentContent > table, .attachment__body {
    font-family: sans-serif;
    text-align: left;
    font-size: 14px;
    line-height: 20px;
    color: #3F4350;
}

.messageAttachmentContent .attachment__author-icon {
    width: 14px;
    height: 14px;
    margin-right: 5px;
    border-radius: 50px;
    vertical-align: middle;
}

.attachment__author-name {
    opacity: 0.6;
}

.messageAttachmentContent p {
    margin: 0;
}

.attachment__title {
    padding: 0;
    margin: 5px 0;
    font-size: 14px;
    font-weight: 600;
    line-height: 18px;
}

.attachment__image {
    max-height: 300px;
    border: 1px solid transparent;
    margin-bottom: 1em;
}


.attachment__thumb-image {
    max-width: 100%;
    max-height: 75px;
}

.attachment__thumb-container {
    max-width: 80px;
    width: max-content;
}

.attachment__wrapper {
    width: 100%;
    display: flex;
    flex-direction: row;
    gap: 12px;
}

.attachment__body {
    flex: 1;
}

.messageAttachmentContent > table.attachment__footer-container {
    color: #a3a3a3;
    font-size: 12px;
}

.attachment__footer-icon {
    width: 16px;
    height: 16px;
}

.attachment__footer-container {
    color: #a3a3a3;
    font-size: 12px;
}

.messageAttachment_title {
    padding-top: 1em;
    font-weight: 600;
    margin-bottom: 4px;
}

.pretext h1 {
    font-size: 28px;
    line-height: 32px;
}

.pretext h2 {
    font-size: 25px;
    line-height: 30px;
}

.pretext h3 {
    font-size: 22px;
    line-height: 25px;
}

.pretext h4 {
    font-size: 19px;
    line-height: 24px;
}

.pretext h5 {
    font-size: 15px;
    line-height: 20px;
}

.pretext h6 {
    font-size: 1em;
    line-height: 1.4em;
}

.pretext > * {
    margin-bottom: 16px;
}

.messageAttachments {
    margin-top: 22px;
}

.messageAttachments h1, h2, h3, h4, h5, h6 {
    font-weight: 500;
}

code {
    padding: 2px 4px;
    font-size: 90%;
    background-color: rgba(63, 67, 80, 0.1);
    border-radius: 4px;
}

.messageAttachments a {
    background-color: unset !important;
    border: none !important;
    padding: unset !important;
}