What type of Excel file does the FO Processor generate using BI Publisher?

Using the BI Publisher APIs from my PX, I generated an output file in Excel format. I then added the file as an attachment to my project. Bur when I tried to view the file, I get the warning message below:

The file you are trying to open, ‘Market_Commitments.xls’ is in a different format than specified by the file extension. Verify that the file is not corrupted and is from a trusted source before opening the file. Do you want to open the file now? 

Please note that I created the file with "xls" as the extension. Can someone tell me what type of Excel file do the FOProcessor APIs generate and how I can avoid this warning?