Excel - File format and extension don't match

Error:

When opening an excel file generated by Adeptia, we're encountering the following issue:

The file format and extension of 'test.xls' don't match. The file could be corrupted or unsafe. Unless you trust its source, don't open it. Do you want to open it anyway?

Cause:

The Excel file used to create the schema is not the same format as the Excel file that is being generated.

Solution:

1) Create the file as xslx

or

2) Re-create the excel schema with an xls file