If you can open it with an editor and see its content I'm not sure it's
a package's fault. An ascii file doesn't have special properties. To be
sure you can make a file by yourself and see what happens. You can also
try to open the file from the browser as a normal file, without going
through the server.
You can look at the encoding, but I would rather look at the character
encoding, instead of the content encoding. Try to set the content type
header to something like "text/plain; charset=iso-8859-1". I'm not able
to go much further; if you want to know more I suggest to ask an http
newsgroup.
If you are using IE6 you can also try another browser. Apparently there
are some problems with the text/plain header. I just found this by accident:

http://support.microsoft.com/default.aspx?scid=kb;en-us;329661

In the worst case I would follow Chris' suggestion. Maybe Ethereal is a
bit low level for what you need, but you should check at least the
response headers coming from the server.