There is only one file type that uses the .TWBX extension:
Tableau Packaged Workbook
A TWBX file is a data file created by Tableau Desktop, a whiteboard data visualization and analysis program. It contains the .TWB workbook plus all workbook data, including the external source data. TWBX files are used for packaging all data into a standalone file, which can be shared with others.
You most likely will only encounter a TWBX file if you use Tableau Desktop or Reader. The file can only be opened and edited with Tableau Desktop, and can only be viewed with Tableau Reader, which is available for free.
NOTE: TWBX files differ from TWB workbook files, which only store references to the external data sources, not the external data itself.
Updated: August 3, 2019