[dylan604]

Thumbnails might work for a folder of images. However, QuickLook will also allow you to preview a video, Word Doc, PDF, spreadsheet, and text files including source code. It's honestly my favorite feature of the OS.

Trying to attach a file to an email, but not sure it's the right one? QuickLook allows you to view the document in the Open dialog. Once you use it, it is something you will just accept as natural and only notice it not being available on other OSes.

[mindfulhack]

Linux Mint Cinnamon has this feature, with package `nemo-preview`. It even plays back actual video when spacebar'ing on an MKV file, something I can't do in macOS!