Google Drive Spreadsheets. It's just a webpage, no-one even needs to download a file with any extension. Plus, people can comment in-line if you want.
so you locked into some elses system that only works when online
1) You can download XLS backups, 2) We are talking about a solution for sharing spreadsheets on an
online forum, And 3), it actually works fine offline if you choose the "work offline" thingy.
So all in all, I'd claim your objections pale in comparison to the disadvantages of XLS or ODS attachments (specifically, the need for every single person viewing your post to clutter their Download folder with your file, and install the right software.) PDF, text, or HTML tables are perfectly suitable for a forum, but of course you lose the editability/interactivity.