I don't have a good solution, Eric.
I started disallowing attached files in discussions because few people saw them. I do not know whether it counts against their quota or not. The external website doesn't provide an audit trail like Canvas does, but both give you the issue that the student could delete the content before it was graded. Back in the days of web development, we were told to always shrink the size of files down. Now people aren't given that choice, have 4K resolution on their laptops, and wouldn't know how to shrink it anyway. My 10 year-old camera takes 4-5 MB jpg images, so if I uploaded 12 of them across the lifespan of Canvas, I would be past my 50 MB limit. Resolutions have increased since then and not everyone saves the image in high resolution like I do, but still ...
Some students use the Google and Microsoft integrations to embed documents in their discussions and that shouldn't count since nothing was actually uploaded, but students would see that more for documents than images (I haven't tried it with images). That had been problematic for me because of browsers blocking third party cookies. I think I finally have enough of them unblocked so that I can see everything that students submit. At least, it hasn't been an issue in the last year.
The biggest offenders for us back in April 2017 (the last I did any research) were people putting videos in a conversation attachment. There was a feature idea (now in cold storage) about this: https://community.canvaslms.com/ideas/8405" modifiedtitle="true" title="Generate error message for u.... I gave some SQL against Canvas Data that would identify issues.
Also check out File Size Limit for File upload question in Quizzes where Erin Hallmark explained things and then came back with a succinct statement. "File upload limits should not apply when uploaded as a file submission." Discussion images are not a file submission and so that statement doesn't apply, which means they can count against the quota.
My concern is how the Adobe Scan is getting the files into Canvas and I haven't checked that one out, but I did have a student say that he couldn't attach his file from Adobe Scan with the mobile app but if he airdropped it to his laptop he could upload it from there. That makes it sound like Adobe Scan is uploading it first and then attaching it. We've been telling our students to use the Adobe Scan since going on COVID-19 shutdown, so this may be a bigger issue.
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