I am not sure I quite understand what you need.
If dealing with an image file that is to be down loaded jD will, with appropriate configuration settings, automatically create a so called 'screenshot' and a 'thumbnail image of the main picture. This allows previewing the downloadable image.
If the downloadable file is a video video or a sound file you can add a separate, probably shorter, file as a preview or jD can make the actual video or audio file its own preview.
There is no specific preview for other 'text' type files such as pdf, doc or similar. The imagemagick preview of the front page of a pdf file is generally rather disappointing in my view unless that front page is essentially a picture itself.
Generally with these type of files the best way is to add to the description but that is a manual operation. Alternatively you could create a picture of part of the 'text' file and upload that as an image as a screenshot and associate it with the Download.
If you could clarify for me,perhaps with an example, I might be able to advise you further.