Fix image display on jupyter

This fix is for bug https://nexedi.erp5.net/bug_module/20160212-76EAAC/view?ignore_layout:int=1 where the image base64 string was only showing. The error in this case was that the mime_type of content code_result was not being updated by the kernel before display.

Reported by : @Tyagov Please review: @Tyagov @kirr

For testing, you can use this jupyter instance with updated kernel running : https://[2001:67c:1254:e:8b::335d]:8888/tree

There are also some example notebooks in the aforementioned jupyter.