Edit/Move or Copy Creating .xls on Tab
 

For some reason, when I email an Excel file to my other office and they open
the file, when they go to copy a tab (by doing "Edit/Move or Copy") and they
click on the name of the tab to rename it, it automatically pops up with
".xls" before the file name. It will not save the name they type because it
says there's an error in the name file. It won't let them delete it or type
over it. I don't understand why it does that in the first place. My boss
thinks it has to do with the file being emailed. Is that true? How can we
avoid this happening? Thank you so much for your time and assistance; I
greatly appreciate it!