There is only one file type that uses the .TAX08 extension:
TurboTax 2008 Tax Return File
Tax return created by TurboTax 2008, a tool used to prepare and file taxes for the 2008 tax year; stores 2008 tax forms and tax data entered by the user; used for filing taxes during the 2009 calendar year for the 2008 tax year.
TurboTax 2008 tax return files rarely use the ".tax08" extension. Instead, they much more commonly are seen as .TAX2008 files.
Updated: October 1, 2010