When I receive an .xlsx file attached in an email, and click on it, the program wants me to save the file, instead of just opening it right away. How can get files to just open up in Libre instead of saving first?
Well, you don’t say what your Operating System is, or your e-mail program, or even which version of LibreOffice you are using, so let us start with a simple question. Are you right-clicking or left-clicking on the attachment??
Your question is off-topic. Refer to your e-mail client manual.