Do you need to read content from the .xlsb file or can you ignore it?
If you can ignore the .xlsb file you might look at creating a dataset from a folder and create a glob based inclusion rule for *.xlsx. This should treat all 6 .xlsx files as one dataset leaving out the *.xlsb file.
The documentation seems to be a bit think on this point. That said I use this approach often. One caution when using this approach the worksheet names in all of the .xlsx workbook file must have exactly the same name. If the worksheet names are different in every workbook you will have problems with that approach.