PowerPivot ( what is powerpivot? ) is stored within Excel files and as such are just normal workbooks, and there is no PowerPivot file extension as such. However, it is important to note that not all Excel file types will save the PowerPivot information and you should only use the approved extensions.
Firstly, any Excel file extension based on Excel 2003 or earlier will not store the PowerPivot information. Also, any extenstion that is not native Excel (open source files etc.) will not store the PowerPivot.
The file types you can use when saving a PowerPivot are: