In DataAssist you need to create your upload page. Now DataAssist does not create a file upload field. So just stick to the default Text field.
Once you are out of the DataAssist wizard you need to change the text input field to a file upload field (Insert>Form>FileField).
Once you've done that you can use the UE Server Behaviour>Webassist>File Manipulation>Upload behaviour to specify where you want to upload the file to.
The DataAssist wizard would have created a insert behaviour. Double click this and find the Filefield field and then click the lightening bolt. Find the Server Entity for File Upload (This is a WebAssist specific thing that's been added to your bindings). Click the one called Server File Name. Then just OK out of the dialogues.
That should be it. A very rough guide.
More specific help can be found in the help documentation. This is no longer that easy to find. The reason is that the file upload was originally a separate extension called Digital File Pro. The following link to a PDF describes how to do this in detail. This is OLD documentation that refers to digital file pro. All you need do is realise that it's now called "file manipulation" and you'll be good to go.