Netscape is aware of the behavior you describe. Netscape Navigator 3.01 remedies the situation. Version 3.01 of Navigator is available for download on the Internet from Netscape.
In the meantime, if you have difficulty using Navigator 3.0 to download files/attachments, you might consider another method of connecting and downloading such files. Try ftp client software such as Fetch, or another web browser, or another email program such as Eudora.