@compass That's strange. All code referencing the route in that error was removed in 3.5. What is the build hash listed in the bottom left of the About Mattermost modal that can be accessed from the main menu in the top-left corner?
Also, I double checked and both routes you listed at the bottom are actually correct.
@meril Unfortunately, I don't know how you could get those files back without doing it manually. Prior to 3.5, the only reference that we stored to uploaded files were as attachments, so Mattermost won't have any explicit references to them any more. One of the reasons for those changes were actually to track them independently so that we could potentially support that case in the future.
For inserting images into the post going forward, we have a community member working on some improvements to the thumbnails like displaying a larger preview when only a single file is uploaded (link to Github pull request), which may get in for 3.7 if it's completed in time.