I see the ability to ignore file extensions in the "find in files" menu of the GUI. Can I do this with script using the UltraEdit.frInFiles?
In help of UltraEdit there can be opened via Index tab the page Scripting commands which lists and briefly explains all UltraEdit specific objects with their functions (commands) and variables (properties).
It is also possible to open in ribbon mode on ribbon tab Layout the check box item Tag list. In toolbar/menu mode with contemporary menus there is in menu Layout the menu item Tag list. And in toolbar/menu mode with traditional menus there is in menu View the submenu Views/Lists with menu item Tag List. Click on this item to open the tag list and select the tag list group UE/UES Script Commands.
The help page as well as the tag list don't list a property for specifying a list of semicolon file names/extensions to ignore which answers your question.
So the UltraEdit.frInFiles.searchInFilesTypes="..."; property must be used to specify with multiple semicolon separated file name wildcard patterns using ? and * like on Windows command line the include filter which excludes the files which should not be found.