Excel

Why isn't .xls a valid uploadable file extension?

Is it possible to get this changed so I can offer an excel spreadsheet for public use?

They probably dont want people uploading Excel sheets with harmful macros built in that will totally screw up your Excel settings.

"Why isn't .xls a valid uploadable file extension?"
Any chance of this happening?
I'm not sure if a "harmful macro" could "screw up Excel settings" but who knows, malicious excel users can be creative.

Note: It's nice to be able to attach files to posts. I wonder if it's feasable to implement an automatic "Virus Scan" to attached files? So far I've been lucky by not downloading some files because other users caught keyloggers/viruses first.

You can zip it and then upload it
http://www.winzip.com/

You can use CSV if you just have tab data. Excel and Open Office should handle that file type just fine. No macros there.