How to change the SQL Server Backup file (.bak) file default icon?
On Windows 10 


Right click on the .bak file
Choose properties
On the General tab, click the Change... button next to Opens with:

Under Other Options choose More apps 

Scroll to the bottom and click Look for another app on this PC

Go the location of Ssms.exe.  Mine was located here:
C:\Program Files (x86)\Microsoft SQL Server\110\Tools\Binn\ManagementStudio
Choose Ssms.exe
Click the Open button and then the Save button


It should now be associated with SQL Server Management Studio and have the icon you'd like