Backport PR
#8132: Only split lines on text output...
Backport PR
#8132: Only split lines on text output
rather than all mime-types, which includes b64 output.
This doesn't change how files are read, so it is still valid for any mime-type data to be split on disk. It just changes which mime-types IPython.nbformat splits on write.
/cc @shoyer who reported the issue on twitter
