You may have some incorrect expectation of what the
uuendcode ... | mailx ... command is supposed to do.
In the world that uuencode was designed to operate in, there is no such thing as an attachment, because MIME doesn't exist yet. A person who receives a message with uuencoded stuff in it is supposed to see it like this:
From: Alice <email@example.com>
To: Bob <firstname.lastname@example.org>
Subject: Those files you requested
Here you go:
begin 644 something.tar.Z
The recipient is supposed to recognize that the lines from
end are a uuencoded file, and pipe those lines (or maybe the whole message) to
uudecode, then look for the output file in the current directory.
It's a lot like decoding MIME attachments, but a bigger chunk of it takes place in the brains of the users.
Some modern mailreaders might automatically recognize uuencoded contents, and use a single user interface to present both uuencoded and MIME-attached files. I wouldn't expect any classic unix mailreaders to do that, because it misrepresents what was actually sent, which is a plain text (non-MIME) message.