I’m pretty sure it does not have to do with spaces.
And this is only with .pdo files, other images and files work fine?
What happens when you try to upload a .pdo file to the media library in the admin, does this work?
I’ve been looking and I can’t really seem to find any information about .pdo files in WP.
At this point I’d just be guessing, but it appears that there is something interfering with the upload. My next step would be to start deactivating plugins, or to see what happens on a test installation with the default theme and only ACF installed, and of course the filter to add the mime type.
