The Canon R5 and R6 became available in late 2020 and create CR3 raw files. I discovered RT 5.8 could not see my files. I managed to understand the instructions to solve this, with some difficulty, that is simple to clarify.
"Add CR3 to the list of parsed extensions in Preferences."
What this means is when you open RT look down the bottom left edge of the screen and just below the ? icon. The icon with 3 dots that look like a gear shift thingy is Preferences.
Click on it to open the Preferences Panel. Click on the File Browser Tab to see the File Browser/ Thumbnail Options panel. Scroll down the right hand panel which lists file extensions and blue tiles with tick marks. Click in that panel to reveal scroll bars and navigate down the list. At the bottom you can Add an extension such as CR3. Finally click the + button.