Maybe someone else knows better, but I am not aware of any file-sharing program that gives you a preview of a file before you begin downloading the file.

Partially downloaded files, particularly video files will enable you a short excerpt of a % of what you have already downloaded if you choose to preview the file.

LimeWire 5 and LimeWire Pirate Edition have libraries for images that give you a Thumbnail preview of what the image looks like. But these are files that have already been downloaded or added to shares/library.

I previously used the earliest FrostWire versions, have tried Frostwire 4.18, 4.20 and 4.21 but do not recall seeing any preview option for image files that have not yet been downloaded. I've also tried several other gnutella programs which lack such an ability.