Upload a picture or a video to be sent later with an answer. The endpoint returns a short-lived access token that must be provided in the send answer payload as attachment.access_token. Allowed extensions are: 'aac', 'amr', 'aspx', 'avi', 'avif', 'bmp', 'c', 'css', 'dng', 'doc', 'docx', 'eml', 'gif', 'heic', 'html', 'ics', 'jfif', 'jpeg', 'jpg', 'keynote', 'm4a', 'mag', 'mhtml', 'mov', 'mp', 'mp3', 'mp4', 'msg', 'numbers', 'odt', 'ods', 'ogg', 'opus', 'pages', 'pdf', 'php', 'png', 'ppt', 'pptx', 'psd', 'rtf', 'sdocx', 'svg', 'tiff', 'txt', 'vcf', 'wav', 'webp', 'xls', 'xlsx', 'xml' .