Editting .msg file - adding attachment

[TriQ200]

I am editting a .msg file in the file system. I can open it in Outlook and
edit the contents and edit any attachments and when I save it it saves back
into the .msg file. However, when I insert a new attachment and save it, the
new attachment is not saved back to the .msg file. When I reopen the .msg
file the new attachment is not there. Instead a new email appears in my inbox
which is the contents of the .msg file with the new attachment.

Why can I not open a .msg file, add a new attachment and save it back to a
..msg file? Why does the message appear in my inbox instead?