The file formats for resources that can be displayed inline in the notebook viewer are:

.doc
.docx  *
.xls
.xlsx  *
.ppt
.pptx  *
.pdf
.htm  **
.html  **
.jpg
.jpeg
.png
.tif  ***
.tiff  ***
.bmp
.gif

* May not be available for installations with Power Mac servers, as there is no official OpenOffice.org 3.x release that supports that platform.
** These files are repackaged into a format that is viewable inline
*** There are some flavors of TIF/TIFF images that do not work.