Ipe image format

Hello,

I'm working on a program that exports Ipe files. Now I try to insert
bitmaps via the <bitmap> element and referencing it via <image> later
in the file. What I did not find out yet is what format the bitmap is
encoded with.

Since I did not find any information on the Web, I took a look at what
Ipe will produce. When I use Ipe to create a file with a bitmap, I get
encoding="base64", Filter="FlateDecode", BitsPerComponent="8",
length="x"

I am able to decode the base64 string to some binary blob of length x,
but what is the format of the decoded string?

Thank you
Sebastian