workaround for no content-type of image bug

If the ending of an image is upper-case, fop 0.20.5 throws error
message: 

Error in Xobject : Error while loading image http://.../name.GIF : class
java.net.UnknownServiceException - no content-type

and the PDF is faulty. Every lower-case one works fine. How can I solve
this??! Setting the type manually by "<fo:external-graphic
content-type='image/gif'>" makes no difference :(

Thank you,
Torsten