To add a file at the job level select the "Upload File" button in Files tab.

In the "Add File" dialogue box you will notice that there are two steps.  The first step is to select a parent folder and the give the file a name.  (If you are a Client Portal user you can also decide whether to share the file with your client by selecting the "client visible" tick box").  The second step is to upload a file.  

It is important that you follow this order as uploading a file first will not allow you to add a description and/or mark the file as "client visible" without making an edit.