If I attach a plain text file "test.txt", and then want to view it, I am prompted to download it first!
HTTP/1.x 200 OK
Date: Wed, 22 Feb 2006 14:41:53 GMT
Server: Apache-Coyote/1.1
Last-Modified: Wed, 22 Feb 2006 14:41:46 GMT
Etag: "1140619306000"
content-disposition: inline; filename="test.txt"
Content-Type: application/x-httpd-cgi;charset=UTF-8
Content-Length: 12240
Connection: close
If I did want to download, I'd have right-clicked and chosen "save link as..."
The strange thing is that for .html pages this works fine.
This was implemented a long time ago, probably in 2.0 or 2.1. Go to Attachments, and click 'Edit' next to the one you want to change.