The first thing we’ll need to do is create a custom field. Click Settings on the left menu and then click Custom Fields on the top Settings menu. 

Next click Add Custom Field.

In the popup that appears, choose File Upload.

And in the next window, give your new field a name and a placeholder name. As you can see, I've simply named the fields “Files” for our example.

You can also choose what types of files can be attached, or select Any to allow all file types.

To attach multiple files, enable the toggle and set the maximum number of attached files. 

Lastly, click Save.

Let’s see how it works. Click Contacts on the left menu and open an individual contact record.

In the contact record, click the Additional Info tab on the left, and you'll see a new option to add files when you scroll down. 

If you click the Add File button, a local file browser is opened. We're going to select this test PNG.

And after the file has uploaded, click Save.

Now our test PNG has been attached to the contact for easy reference.

And that’s all there is to attach files to your contact records!