The change doesn't require the "random refactoring" I feared.

Assuming others think this is a reasonable idea, a problem arises in that WARequest doesn't implement #mimeType. I'd propose refactoring WAExternalFileLibrary>>#mimetypeForRequest: and WAFileLibrary>>#mimetypeFor: 

Does that seem a reasonable course of action?

Instead I implement 

| path |
path := self requestContext request url path.
^ (path notEmpty and: [ path last includes: $. ])

and modify PRPierFrame>>#notFound to read:

"This method is called whenever the current request cannot be resolved to a structure."
self isFileRequest ifTrue: [ self requestContext respond: [: response | response notFound ] ].
self context: (self context
structure: self context root
view: IZNotFoundView)

Anyone think this is a bad idea? If not I'll check it in.