This is the code I've tried - I've got the file extension stored in Obs_Image_Type and I'm trying to test its value for MOV or MP4 - if true, I want to set the image control source to display an error jpg, otherwise display the actual image.  It does not display the error jpg.

Hold on, still working on it.   the above comment was wrong - I think I can figure this out. Expression seems to be working, something else is not.

You could do the opposite:
Legacy Tip: Remove file extension

Alex - Thanks for the reply, but what I needed to do was determine if the control source was a file sufficiently small for the image control to display - video files are too big and cause error.   My problem was that I had added a field Obs_Image_Type to the source table and saved that, I had added the code to save the file extension in the VBA in my form which let me browse and select a file, but the extension was not actually being saved.   I've seen this before - if I add a field to a table, in order to use it on a form I have to add the field to the form, even if it is not to be displayed; then, I can delete the field from the form, but Access now 'knows' about it, and I can use it in VBA etc.   I've got it working as I want, with the expression shown above, plus the VBA to estract and save the file extension.