Extend filetype support to e.g. PDF, video, etc.

[pond]

A Mac App Store reviewer writes:

by Eugene – Feb 17, 2018
Wish it could make previews not only for photos, but also for videos, pdf, documets, etc. As it was made in Windows Explorer.

In newer OS X versions with faster machines and SSDs quite common, this becomes viable on large folder sets. Consider this addition (via delegation to the QuickLook generator for each thumbnail?) - probably related to #1 if overhauling the generator, as it'd make sense to have extended filetype selection implemented as an additional generator option.