Hi Subhash,
As Miguel said, you could also enable document management for custom entities.
This will add the Document tab to the Related tab in entity form.

Its function is the same as the File tab of the OOTB entity.

It does not appear on the form by default. Users need to click Related to see it.
If you create a subgrid for Document Location (Regarding), the subgrid will be displayed on the form by default. But there are not as many functions in the subgrid as the Document tab.
So both solutions have their own pros and cons, please choose the one that suits you.