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.