Image thumbnails and download working but previews are not

[Isa Hassen]

So we finally finished a major upgrade (after about a year), and everything seems to work fine, except our old uploaded files. Newly uploaded files work fine, but old ones are broken. Here's the kicker though: the thumbnails are displayed correctly, and the download button works. Only the file previews and embeds in course pages are broken! To provide context I've attached a screenshot of the broken image preview at this URL: https://canvas.qasid.com/courses/5/files?preview=52

When I check my JavaScript console, I see this:

GET https://canvas.qasid.com/courses/5/files/52/preview 400 (Bad Request)

However I can't seem to find this 400 error anywhere in my error logs. I've tried looking in my apache error logs as well as the rails production log. No sign of any 400 errors. If the image exists on disk with the right permissions, and the download button works, why on earth would it fail to display in the preview?