How to create a lookup for any object in Attachments Object?

Goto Setup►Objects

Select the Attachment Object

Goto Custom Fields and Relationships in Attachments object

Click on NEW

Select Lookup as the datatype and click on next

Select the object for which you want to create a lookup and click on next

Enter the object name in the field name textbox and click on next

Provide the field accessibility as per your standards

Save the settings