I have an android phone. I have two apps on the phone, both are able to open djvu files (FBReader & EBookDroid). But when I'm trying to open a djvu file in "my file", the android system says that it is unable to find an application toopen the file. How I could set a default application to open djvu files?
This is usually a problem with bare-bones file managers. Switching to a more feature rich file manager should solve it.
Head to Play Store and try a famous file manager. I can recommend Solid Explorer because I own it and use it, and it gives the option of assigning an app for a file type (subject to mime-type limitations).