How do I find all files categorized movies on my disk? In the dialog of "About this Mac", started from below the Apple menu item, the Storage 'tab' says there are 42.43 GB of Movies.

Ok, how do I find all those files that the system categorizes as movies?

In other words - how did OSX find them? 

I can't find them anywhere.

Is there a Terminal ls -al | grep [something] that I can fire off from the root folder to help me find them?
In a Terminal, use the following command:

mdfind "kMDItemKind == '*movie'"


This will output the fully qualified pathnames of files in which the metadata stores used by Spotlight classify it as some type of movie.

Example output from a movie taken on my iPhone and transferred to iPhotos on my Mac, one of the lines outputted by mdls is:

kMDItemKind                    = "QuickTime movie"

"QuickTime movie", which have a .mov extension was just one of extension types that showed as "$some_type_of movie". So you will probably see other file extensions as well.