The warning you're seeing comes from READTABLE; it's probably not related to the names of your files (unless those are being stored in the XLSX file).
Find the call to readtable, and then add the parameter 'ReadVariableNames',false ... e.g.
t = readtable(filename,'ReadVariableNames',false);
This will make the warning go away, but I don't know if that's the behavior you want--the variable names of the table will look like 'Var1', 'Var2', ... etc..
Hope this helps, Jeremy