Drag python file into editor when not working on main.py

[microbit-robert]

When dragging a python file into the editor while not working on the main.py file, the resulting dialog always offers to replace the code in main.py, but not the currently open file, which could be misleading.

[microbit-matt-hillsdon]

This is working as intended and the dialog makes it clear what will happen. The process of uploading third-party modules that don't have a magic module marker isn't very clear and would be good to revisit in future but this aspect probably isn't a priority. Closing.