Files cannot be deleted unless they are not being used anywhere within a showcase - but did you know that files still count as 'in use' if they are loaded into your shareable content, even if they aren't visible within a showcase? Easy to miss.
If you are trying to delete a file and it says it is still in use and cannot be deleted, the best thing for you to do is check the usages.
In the file library, there is a button called "Usages" in the sorting options in the top of the library.
Want a closer view? Sure. It's the small question mark in a bubble.
Clicking this will bring up a list of everywhere where the file is currently being used.
It will state the showcase, and either the hotspot it is loaded into, or the slide - and if it is loaded into your sharing list, it will say that also. That way you know where you need to navigate to to remove it, so you can delete the file.
The usages panel has quicklinks to where it is located - just click the link and be whisked away to that exact point in your showcase.