To view the list of file types recognized by PyCharm, go to Settings/Preferences (Ctrl+Alt+S) | Editor | File Types. If a file in your project is marked with the icon, it indicates that PyCharm can't recognize it. In this case, you can register and configure a new file type.
The lock decorator in the upper-left part of a symbol marks a symbol whose name begins with one or more underscores. Such names are considered pseudo-private and are specially treated by an interpreter to restrict their visibility scope.