If you wish to upload a new document to a Contact Record, first go to Contacts and click on a contact name,

then simply go to Options on the right and from the drop-down menu, click Add Document. This will bring up a dialogue for  you to choose and upload your documents.

If you want to link a document that is already uploaded and in the File Manager, click the 'Other' tab on the contact record. You will see the Documents section at the top. 

To select the relevant document you want to add, all you have to do is click on the drop down option and find the document and press "Link this Document".