Relevant documentation for reference:
https://docs.intersystems.com/irislatest/csp/docbook/DocBook.UI.Page.cls...

AFAICS the documentation says nothing about the relationship between OriginalFilename property and %f specifier.

Argh. Ens.StreamContainer#StreamSet silently overwrites OriginalFilename property.

Filename (%f) is empty:

// in is a character stream

#dim exportRequest = ##class(Ens.StreamContainer).%New()
set exportRequest.OriginalFilename = "foo"
set status = exportRequest.StreamSet(in)

Filename (%f) is "foo":

// in is a character stream

#dim exportRequest = ##class(Ens.StreamContainer).%New()
set status = exportRequest.StreamSet(in)
set exportRequest.OriginalFilename = "foo"