If it matters, I use Flask-Security, Flask-SQLAlchemy and Flask-Admin in my application.
How should I design my application so that I can connect a row in the database with one or more files, e.g. a user to their profile pictures?
If only one file/picture is required, it should work by adding another column. But if you want to associate multiple files to a specific user, you can create a new table for the relations:
files table:
ID | USER_ID | FILEPATH
===================================================
1 | 1 | somewhere/on/the/server.jpg
2 | 2 | another/file.py
3 | 1 | second/file/of/user/one.py
The USER_ID column is a foreign key that references to the primary key in the users table. Take a look at the docs of Flask-SQLalchemy.