"crash.log" is just a name for the text file where we store app logs. It should be almost empty unless you enabled verbose logs or you got a crash.
".log" is also an extension that KOReader can understand and render. That's why it is enumerated as a file extension to associate with the program.
If you associated all extensions with KO you'll get the crash.log on your home, just like any other supported document.
Removing the association will remove the document from home, because it is not part of the registered extensions the PB FW deals with.
TL;DR: we should change instructions in
https://github.com/koreader/koreader...etBook-devices and recommend to associate the file extensions the user wish to open in the app, not all of them.