There are many formats, not only encrypted but also compressed file formats with an entropy as high that it is basically undistinguishable from random data.
As zip files don’t get blocked, I’m pretty sure MS has an internal table with all the different file extensions and their usual shannon entropy.
Either this table is self-learning, then the problem will vanish once enough users have uploaded .c9r files and OneDrive has learned that this is an encrypted format, or users complain to MS about this fact.
If you want a fast solution, the latter might be the best option right now. 