Display custom type with PNG support

How can I use imshow to display an image of my custom type which has support for the image/png MIME type via Base.show?

Is it not supported, or is imshow not the appropriate function for this? Or does ImageView support some kind of AbstractDisplay, so that I could write display(ImageViewDisplay, Foo())?

Minimum non-working example:

julia> using ImageView, PNGFiles, TestImages

julia> struct Foo end

julia> Base.show(io::IO, ::MIME"image/png", x::Foo) = PNGFiles.save(io, testimage("mandrill"))

julia> imshow(Foo())
ERROR: MethodError: no method matching size(::Foo)

Closest candidates are:
  size(::Core.Compiler.StmtRange)
   @ Base show.jl:2774
  size(::Base.ExceptionStack)
   @ Base errorshow.jl:1018
  size(::Base.AsyncGenerator)
   @ Base asyncmap.jl:389
  ...

Stacktrace:
 [1] axes(A::Foo)
   @ Base .\abstractarray.jl:98
 [2] roi(A::Foo, dims::Tuple{Int64, Int64})
   @ ImageView C:\Users\jwortmann\.julia\packages\ImageView\Gg1GW\src\slicing.jl:45
 [3] imshow(img::Any; axes::Any, name::Any, aspect::Any)
   @ ImageView C:\Users\jwortmann\.julia\packages\ImageView\Gg1GW\src\ImageView.jl:277
 [4] imshow(img::Any)
   @ ImageView C:\Users\jwortmann\.julia\packages\ImageView\Gg1GW\src\ImageView.jl:274
 [5] top-level scope
   @ REPL[13]:1

Start with the imshow entry point in src/ImageView.jl around line 274 and the roi logic in src/slicing.jl around line 45. Reproduce the Foo() example and determine the display contract for custom image/png values, including whether an AbstractDisplay path is relevant. Done should be a tested or documented resolution for the example.