The data type of the field is "File", and I'm using the "upload file" form control.
It does show the GUID, because when I add a trailing / with some random text, the image will render.
Not sure what's going wrong here, since I've done the same thing in K8.2 and it was working like it should.
something like this works, mind the + "x" at the end :
<li style="background:url(<%# GetFileUrl("SliderImage") + "x" %>)">
It seems that it has something to do with the Files friendly URL extension in K9. I removed the default value (.aspx) to empty. This works fine in K8 but not in K9. Even the example sites won't show images if I remove the .aspx.
In K8 it adds the filename behind the "/", ex:
<img src="/SiteName/getattachment/512b1b3c-8f26-493f-a0b0-b0c35174d996/Plane-2?maxsidesize=200" alt="">