So I did some research on this.
The alternative used in GNOME to send files as email attachments from
Nautilus is the org.freedesktop.portal.Email desktop portal.
Unfortunately this uses a non-standard extension to the `mailto:` URI
protocol to include attachments: an extension that Thunderbird, the
default mail client in Ubuntu Desktop, does not implement -- not for
lack of interest but for security concerns:
https://github.com/flatpak/xdg-desktop-portal-gtk/pull/447#issuecomment-1761589783
Furthermore, if this were to actually work for sandboxed applications
(which currently it should not), a malicious application could
effectively read the full host filesystem by just opening `mailto:` URIs
on itself.
nautilus-sendto itself is a fairly trivial program: it calls a
client-specific commandline, for every mail client known to it. That of
course means that it requires continuous maintenance for adding support
for new mail clients, but on the other hand there is no real standard
interface available to implement instead.
I think we'll keep using it in Ubuntu Desktop (through a patch in nautilus)
