Adding an optional thumbnail for a multimedia file

[bdub]

How do I add an Optional Thumbnail to a pdf file after the file has already been uploaded?

I have the default icon for that particular file type assigned, but would like a different one in this instance.

To better explain my question, ... When I upload a pdf file, I am given the opportunity to upload an Optional Thumbnail. If I do not upload one at that time, but decide to add one later, how do I do that?

I can't seem to be able to find that screen again.

Thanks

[Chuck S]

Edit the photo and there is a spot on that screen