Bishwas-py the "preview" template included in this extension currently only works for images, by directing linking to the file inside of an img
tag. There's no thumbnail generator.
If we go add some sort of thumbnail generator in the future, then it would be feasible to add thumbnail support for non-image files.
As discussed previously, using a combination of the simple URL template in fof/upload and a custom MediaEmbed template, one could implement a custom preview script on their forum.