There is only one file type that uses the .TAX12 extension:
TurboTax 2012 Tax Return File
Tax return created with TurboTax 2012 tax preparation software; contains United States tax forms for the 2012 tax year along with data entered by the user; may be encrypted, which requires a password to open.
NOTE: While TAX12 files can be opened with TurboTax, most TurboTax 2012 tax returns are saved with a .TAX2012 file extension.
Updated: October 31, 2016