Your Odoo database should have a linked folder 'filestore' on the server. For example, when you create a full Odoo backup, in the made archive you can find both the database (sql) and a directory with attachments.

Thus, when you make a migration you should not only start a code on a new database, but you should also copy this filestore. For details have a look at this old but still useful forum questions with a plenty of hints - https://www.odoo.com/fr_FR/forum/aide-1/question/where-are-document-attachments-stored-529

Take into account that in case you copy records with a script (not starting the same database with a new code), IDs of objects would be different. In such a case mere duplicating of a filestore would not help.