Newbie here

I don't know much about Macs, but what I do when my PC saves them with a wrong extension is just edit the extension to read .xml, and when it saves them with an added extension, I simply remove the addition (i.e. sometimes they save as abcdefg.xls, in which case I the change the "ls" to "ml" and then they work fine; other times, they save as abcdefg.xml.xls, in which case I just delete the ".xls" part and then they work fine). As I say, I don't know if your Mac will let you do this. I also don't know why my PC insists on (often but not always) doing this.