That's never a good idea. Especially with many flavours of Excel files by M$. Simply - Excel and similar formats are not suitable for machine processing. That's why we tend to use standard data exchange formats such as CSV at least.
If you really have no other options - then you have to rely on third party plugins / scripts which will get you raw data out of those files.
P.S. I did Excel parsing for years but nowdays I simply refuse to do it - if someone is that lazy to export from Excel to CSV, then no business with such entities. You learn about attitude as you grow older. Folks working with Java would simply insist on full blown XML, not even CSV.