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Briefly, the MIME type is how the server informs the browser what kind of data is coming through. For unspecified types, the browser has to guess; IE coming from Microsoft may guess that .xls is Excel, but Netscape might not-- and attempt to display the file as plain text.
Depending on what you're trying to do (i.e. if you're working mostly on the viewing end as opposed to the posting end), you can manually instruct Netscape to open .xls files using Excel by adding the MIME setting locally under Edit->Preferences->Navigator->Applications (NN4) or Edit->Preferences->Navigator->Helper Applications (NN6+).