Supported readers include:

Note that showing .xls and .xlsx means the reader doesn't care whether it is .xls or .xlsx but if you use .xls you are subject to the limitations of that very old file format, but if it works for you - the reader is fine with it.

The uploaded file format must match the configured reader type. Unsupported formats cannot be processed.

XLS, XLSX, CSV or JSON?

If human, likely XLS or XLSX is the best. If computer, CSV or JSON is the best. Why not XLS/XLSX? read on!

But note that Excel may corrupt your data. For example, if you enter FE-2 as a string value, perhaps a part ID, Excel will change that to "February 2nd" ... a huge shock to many people. (It is such a severe problems that there are some chemical names that have been around for 100's of years that have been changed in the scientific community due to the desire to store data in .xls(x) format that kept getting changed into dates!

While we know that XLSX is heavily used when coming from other computer systems, if the 'other' computer system has the ability to give .csv instead of .XLSX - choose .csv because of the above problem. If you have a choice, XLSX is fantastic for data entered by humans, csv and JSON is excellent for data coming from another computer system.

But having said that, we know that, informal statistic, more than 90% of the time, people use .XLSX these days when exporting from another system, so despite the above, .XLSX remains the favorite for importing regardless of the source.

OK so what about this .XLS vs .XLSX?

When it comes to data transfer, XLSX is the better choice compared to .XLS. It is the modern Excel format that supports more rows and columns, creates smaller files, and is more reliable when files get corrupted¹. It has been around for a very long time now. The only reason to use XLS is if you need to send a file to someone running Excel 2003 or an older system that only accepts XLS, and when coming back, if the above user is sending the file back or if the file is coming from an older system that can only expert in .XLS, not in .XLSX (but then ... see if it can export in .CSV).