How do I find all files categorized movies on my disk?
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.