It is not possible to create a custom field with lookup or master detail to Attachments on a custom object because every custom and standard object contains a related list with Notes and Attachments. You can add this field to your custom object by checking a checkbox at the time you are creating a custom object.
After that go to the page layout and click Edit. Select Related list and drag the Notes and Attachment tab to the related list section and click on Save.
It is not possible to create a custom field with lookup or master detail to Attachments on a custom object because every custom and standard object contains a related list with Notes and Attachments. You can add this field to your custom object by checking a checkbox at the time you are creating a custom object.
After that go to the page layout and click Edit. Select Related list and drag the Notes and Attachment tab to the related list section and click on Save.
Hope this may help you.
Regards,
Akshay