Handle PATCH requests to /partial-content.txt; refactor FileHandler and ...

...subclasses
There seem to be a lot going on in this class. I would take a look at this class with the SOLID principles in mind, particularly the SRP and the ISP.

This is definitely related to my comment where I said the "inheritance seems a little strange" in PR#3 but not identical. I see other mixing of concerns that do not seem to be related to the inheritance structure.