How to make Mac windows show extensions for video files?

Are you sure these files actually have extensions? What does it look like if you Get Info on it? In the BeforeTime, file types and creator codes were stored in the file allowing the Mac to tell what kind of file it was and what application should open it without needing an extension. I mention this because it sounds like you might be working with very old footage.

By "windows show extensions for video files" do you mean Finder? Do you mean the FCP Event Browser? Are you asking about Final Cut Studio or modern-day FCP?

In the FCP Event Browser, if you select List View (OPT+CMD+2 toggles), then right-click on the column header, you can select "File type." That will show MP4, Sony MXF, etc. It's not exactly like the file extension but it's similar.

The items in the Event Browser are called "clips" which is a little different than files. E.g, you can rename a clip to whatever you want, and it retains the same filename. The clip name originally chosen by FCP upon import is taken from the filename, but it's not really the filename, it is a metadata item.

If you want a detailed list of all clips and the corresponding files, there is a third-party utility Final Cut Library Manager (now called "Arctic") which can export a .csv file showing that. It is an excellent utility that does various helpful things.

I wrote a simple Python script which will parse an Event XML file and show the filenames, pathnames and whether each one was imported "in place" or "copy to library." It also has the command-line argument --csv to export as a .csv file instead of printing to the screen.

To use:

• copy/paste the below script to a file named fcp_extract_clips.py
• Grant the .py file execute permission: chmod +X fcp_extract_clips.py 
• In FCP, select an Event in the left sidebar
• File>Export XML
• It automatically handles both .fcpxml and the newer .fcpxmld format
• Syntax example (print to screen): python3 fcp_extract_clips.py MyEvent.fcpxmld
• Syntax example (write to csv file): python3 fcp_extract_clips.py --csv MyEvent.fcpxmld
• If you provide no arguments, it prints command line help

In addition to what terryb said: the name Capture Scratch suggests that these are not individually playable video files, but files used internally by the program (probably when capturing from tape). This would be similar to how render files are used in today's FCP.