I'm at the initial setup screen for Kdenlive and there is an option that says "Install extra video mimetypes". What are extra video mimetypes, and is it necessary to have them installed for Kdenlive to work properly?

Extra mime types allow Kdenlive to recognize *.m2t, *.mts, *.mkv and some other extensions as video files, because the default distro install does not list those as video files.

Kdenlive works perfectly without installing the mime types, but when opening the "Add clip" dialog, some files will not be shown because they are not recognized as video files.

The mime types can always be installed later through the menu: Settings -> Run Config Wizard

regards

After starting Kdenlive an error window is displayed that says "Error - %c File '/home/user/%i' is not readable". What would cause that error?

Probably some permission are false.