There is only one file type that uses the .TAX15 extension:
TurboTax 2015 Tax Return File
A TAX15 file is a tax return created by the 2015 version of Intuit TurboTax. It contains personal information entered by the user and includes federal and state tax forms.
The TAX15 files are an alternative to the default .TAX2015 files created by TurboTax to save tax returns. You most likely will never see a TAX15 file since TurboTax saves tax returns as TAX2015 files by default. However, if you do come across the TAX15 file you can open it with the 2015 and 2016 desktop versions of TurboTax, which is available for both Windows and Mac. After the TAX15 file is opened you can then save the tax return as a TAX2015 file.
You can electronically submit TAX15 files to the IRS and state tax authorities. The "e-file" process is free for federal returns, but most likely requires a fee to file state returns electronically. You can also print and mail TAX16 files if you do not want to file them electronically.
Updated: May 3, 2017