The previous multipart attempt (f74fd07) was rejected by Civi with
"Mandatory key(s) missing: id or content or options.move-file" — the
/civicrm/ajax/rest endpoint on this install doesn't expose $_FILES to
v3 actions; only params['content'] is consulted. Our multipart `file`
part was ignored.
APIv3 Attachment.create has historically been the file-upload entry
point used by Civi's own form widgets and auto-decodes the `content`
field from base64, unlike v4 File.create which stores it verbatim.
Send the same shape we tried first (entity_table, entity_id, name,
mime_type, content=base64) but to v3 Attachment instead of v4 File.
The civi3Upload helper in lib/civicrm.ts is kept in place — it's not
useful for this endpoint but the multipart-POST shape may be needed
later for other Civi entities that do read $_FILES.
Joel Brock
e42e3b70ef