@kayjay51
As far as I know, there is no such file format in Excel.
Since i didn't see any further information about Excel version, operating system and storage medium, I would have to guess.
First I would change the file extension, instead of .xsls I would change this to .xls or .xlsx (depending on the Excel version).
File formats that are supported in Excel
Whenever you save a file in another file format, some of its formatting, data, and features might not be transferred.
If this does not help either, I would recommend inserting the file (without sensitive data) in order to be able to quickly trace the origin of the problem.
Hope I could help you with these information / links.
NikolinoDE
I know I don't know anything (Socrates)
