Review UIs have received some modifications in this release (and in

previous releases) that broke the separation of ReviewUI and

FileAttachmentReviewUI. The former started receiving logic specific to

file attachments, which belonged in the latter. It also provided a

number of functions that made some assumptions about the types of

objects it was working with, when it should have provided stub functions

to overwrite.
In practice, this isn't likely to have impacted extension authors, who

are going to be subclassing FileAttachmentReviewUI, but non-attachment

Review UIs would have required working around the misplaced logic. This

change fixes all this so that the logic is all in the correct classes.
It also adds a bunch of missing documentation, fixes bad documentation

and error messages, implements necessary methods in

LegacyScreenshotReviewUI, and fixes a regression with that UI's

thumbnails that broke clipping of thumbnails.

Unit tests pass.
Manually tested thumbnails and review UIs for legacy screenshots, PDFs,

images, and text files.