After you have uploaded a file, you have the choice of either creating a new entry using that uploaded file, or just displaying the HTML used to link to the file. For example, if you have uploaded an image, and would like to create a new entry with a link to that image in a popup window, select Create a new entry using this uploaded file, and press POPUP IMAGE. If you are already editing an entry, then you won't want to create another new entry (and perhaps lose the one that you are working on); so in that case, click the Show me the HTML button and click POPUP IMAGE. This will display the HTML code for opening the image in a popup window. You can then copy and paste this code into the entry you are editing.

See Uploading a File for more details.