Export ExtraFile from app.models

app/models/__init__.py

@@ -1,5 +1,6 @@
 from app.models.user import User
 from app.models.set import Set
 from app.models.instruction import Instruction
+from app.models.extra_file import ExtraFile
 
-__all__ = ['User', 'Set', 'Instruction']
+__all__ = ['User', 'Set', 'Instruction', 'ExtraFile']