As many of you have found out, everything is not as it seems with the Excel output that gets generated from an RTF template. It has non-Excel like limitations and the files are quite big. Those of you using Excel 2007 will have noticed another niggle. When you open an 'Excel' output you get a warning along the lines of:
The file you are trying to open, '*.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?
This is down to BIP actually generating HTML and setting the mime type and extension to get Excel to open it. This is the cause of the limitations and the cause of the warnings. The limitations can be addressed by using an Excel template more on those in the user docs and elsewhere in this blog. Addressing the warning is going to require a modification to your users' registries.
Huge thanks to Abhishek Gupta for digging up a note from Oracle Support on it and thanks to whoever wrote the original. The note number is 1077728.1. I have uploaded Abhishek's snippet form the note here.
Update: Leslie has reminded me; for she is my Jimney Cricket to my Pinocchio, constantly prodding my conscious :0)
As an additional option in 188.8.131.52 Excel 2007
is a new output type from RTF that generates .xlsx.
Excel 2007 is a new output type available for reports. When
BI Publisher generates the output in Excel XML format (.xlsx). If
you have Excel 2007 or later installed, this option provides the
preservation of layout and formatting.
As noted in the comments, the Excel templates generate binary xls output not xlsx format and therefore have xls limitations in their abilities.