I was doing a customer support chat w AT&T a few weeks ago in Safari. When it was over I clicked the ‘download conversation’ (or whatever) button and got this message. I poked around iCloud Drive a bit but didn’t see any file structure that looked like this. I don’t really need the file, but I am curious as to where it ended up. 
Based on the path, I’d almost expect it to be in a Docker container on a server?
Here’s a couple of “Containers” folders I know of:
Go to Finder, then from the menubar Go >> Go to folder.
Type “/var” to get to that one.
There’s also one in the Library, so in Finder open up the Go menu, hold down the Option key to reveal the secret Library folder in the list and select it, and there’s an alternative Containers folder.
Fwiw, the “Containers” directories on MacOS and iOS aren’t docker containers, as far as I know
I’m pretty sure that’s the Documents directory for whatever app has that UUID. I’d check the various application specific folders in the Files app on the device you were using. (This was on an iPhone or iPad?)
Sorry, I wasn’t paying proper attention to the title, I only read the post.
Looking at my apps in Xcode, I can see ACautionaryTale’s correct.
Additionally, I think that the files in that folder are not going to iCloud, but if you plug your phone into a Mac, they’ll appear in Finder on the phone.
As far as I know you’re right; (on iOS) these containers are the result of sandboxing, which ensures that each App is contained in its own container(s) and cannot access the container(s) of other Apps.
Folder /private/var/mobile/Containers/Data/Application is the location for data containers which hold user/App data for an App.
