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You need to select "File Upload" from the "Basic fields" left column. Please check the screenshot below. If you face any difficulty, let me know.
That's a good advice by Irshad.
Just to add to his reply, in case that you are looking to upload a picture to your form instead of allowing users to allow one, you should use a different field, it is the image field and you will find it under the Basic Fields:
Once you add the field, you will get a window like this one where you will be able to upload an image, select one previously uploaded or enter the URL to an image:
This will display the image in the form.
Hope this helps.