Netscape's News will automatically decode UUEncoded and MIME-encoded attachments. It will sometimes display images automatically within the newsreader.
The down side is that it's a rather poor at handling binaries in general. The more posts you read, the slower the program runs and has been known to suddenly quit with an error message. It also has no support for multi-part binary posts at all.
In the second case, you need to save the file and then open it. If you are on a Windows/Win95 computer, hold down the SHIFT key and click on the link. If you are on a Mac, hold down the OPTION key and click on the link. The Save As window will appear so you can save the file to your hard drive.
If you are going to be doing a lot of work with binary posts, you'd be best off getting either Free Agent (for Windows) or Microsoft Internet News (for Mac or Windows).