There is only one file type that uses the .U12 extension:
UFile 2012 Tax Return
Canadian federal tax return created with UFile 2012, a tax preparation program for Canadian residents; contains Canada income tax forms and data entered by the user; may be printed and mailed or filed electronically with the Canada Revenue Agency using the program's NETFILE feature.
U12 files can be opened directly by UFile and may be imported by the Canadian version of TurboTax.
Updated: April 28, 2013