There is only one file type that uses the .TAX10 extension:
TurboTax 2010 Tax Return File
Tax return file created with TurboTax 2010 tax preparation software; contains U.S. federal tax forms for the 2010 tax year and data entered by the user; may also include state tax forms and related data if a state version has been downloaded.
NOTE: Tax returns created with TurboTax 2010 are usually saved with the .TAX2010 extension. However, the TAX10 extension is also supported.
Updated: September 16, 2010