Hi Ken,
In the end I have come with an embarrasingly simple solution:
UploadAttachments.Items.Add method + AttachmentItem.ClientData property
The simplest, the best.
However, I have a problem when trying to remove from disk an attachment added this way. The file is being used by another process is throwed. I guess the stream used to read it must be closed first, but I don't know where.
Is there a way to achieve that?