def _get_social_page_image_url(file_attachments):
    """Return the URL to an image used for social media sharing.

    This will look for the first attachment in a list of attachments that can
    be used to represent the review request on social media sites and chat
    services. If a suitable attachment is found, its URL will be returned.

    Args:
        file_attachments (list of reviewboard.attachments.models.FileAttachment):
            A list of file attachments used on a review request.

    Returns:
        unicode:
        The URL to the first image file attachment, if found, or ``None``
        if no suitable attachments were found.
    """
    for file_attachment in file_attachments:
        if file_attachment.mimetype.startswith('image/'):
            return file_attachment.get_absolute_url()

    return None