There is only one file type that uses the .T15 extension:
H&R Block 2015 Tax Return
Tax return created and used by the 2015 version of H&R Block Tax Software and H&R Block Online; contains U.S. federal tax forms, any necessary state tax forms for 2015, and personal tax information entered by the user.
The T15 file can be saved and worked on at a later time. Once you finish, you can electronically file the return directly from the H&R Block software or H&R Block Online. The return can also be printed and mailed to the appropriate tax authorities.
Updated: August 30, 2016