The "attachment" command:

Usage: fossil attachment add ?PAGENAME? FILENAME ?OPTIONS?

Add an attachment to an existing wiki page or tech note. Options:

-t|--technote DATETIME

Specifies the timestamp of the technote to which the attachment is to be made. The attachment will be to the most recently modified tech note with the specified timestamp.

-t|--technote TECHNOTE-ID

Specifies the technote to be updated by its technote id.

One of PAGENAME, DATETIME or TECHNOTE-ID must be specified.

DATETIME may be "now" or "YYYY-MM-DDTHH:MM:SS.SSS". If in year-month-day form, it may be truncated, the "T" may be replaced by a space, and it may also name a timezone offset from UTC as "-HH:MM" (westward) or "+HH:MM" (eastward). Either no timezone suffix or "Z" means UTC.