Spreadsheets are not one of the default file types allowed for attachment in Xenforo. Allowed file types include: zip, txt, pdf, png, jpg, jpeg, jpe, gif.
One reason for this may be that malicious macros can be embedded in spreadsheets, a lot like malware. In fact, not only do they pose a risk to the person that downloads the attachment, but it's my understanding that the files may be exploited on the forum server and potentially cause harm there, too. Of course, this is a small risk and your spreadsheets, Case, won't include risky macros, but I've got sketchy people trying to join the forum all the time that I filter out and I'm not sure I want to take the risk.
One thing you can do, within the allowed file types, is to ZIP your spreadsheet and attach the ZIP file. At the end of the day, this probably isn't any safer for the end user of the spreadsheet and I don't know if it's safer for the forum server, but it is what it is.
Another approach would be to import your Excel spreadsheet into Google Sheets, then include a link to the Google Sheet in your post. That way, there is no attachment on the server, you can update the spreadsheet online, and people always get the latest version when they click the link in your post.