[XeTeX] How does xetex handle non-native image files such as eps?

[Yuan Qi]

Does it extract the boundingbox info from the file first and
finish the layout before calling the external program to 
convert the image to PDF or does it convert the file to PDF 
first before starting any layout work? Also, xetex does not 
appear to accept any non-native image files besides eps 
files. What other settings needs to be modified besides 
dvipdfmx.cfg in order to get it to accept other non-native
image files?