My application requires to open the default mail client and to compose a
new message using a specific attachment.

In order to be desktop manager agnostic, I use the command xdg-email.
When gnome is the running desktop manager, xdg-open calls gnome-open,
and gnome-open call evolution.

xdg-email encode the attachment pathname into URL, and gnome-open call
evolution using the same encoded string, but evolution doesn't un-encode
that string and cannot find the specified file.

Example:

$ XDG_UTILS_DEBUG_LEVEL=30 /usr/bin/xdg-email \
    --attach /tmp/allegato5316366537919686103.TIF 
Running gnome-open "mailto:?attach=%2Ftmp%2Fallegato5316366537919686103.TIF"

you may see that xdg-email encode the string and call gnome-open. Then
evolution pop up and display an (Italian) error message: Impossibile
allegare il file «%2Ftmp%2Fallegato5316366537919686103.TIF» a questo
messaggio (It's not possible to attache file ... to this message). The
error message detail says "file not found".

I tried calling directory evolution using this mailto URI and I get the
same error. The command I write is:
$ evolution "mailto:?attach=%2Ftmp%2Fallegato5316366537919686103.TIF"

Thanks,
Giuseppe Sacco