Hello!

I have a problem the documentation doesn't seem to help with.

I am creating an epub from markdown, but I want to replace the titlepage 
with a pre-made image.

The approach I thought I would take was this:

1)  I can't quite work out how to get at the titlepage from a filter, since 
that's specific to the epub writer.  So instead I use a custom template for 
epub3 output and replace the default titlepage with the following: 


[...]
$if(titlepage)$
<section epub:type="titlepage">
<img src="../media/titlepage.png"/>
</section>
[...]


 2) Run pandoc with a key 'epub_title_page_image' in the metadata, pointing 
to the pre-made image. I thought that by using this lua filter, the image 
would be added to the mediabag.


function Meta (meta)
  local titlepage_path = 
pandoc.utils.stringify(meta["epub_title_page_image"])
  local mimetype, contents = pandoc.mediabag.fetch(titlepage_path, ".")
  pandoc.mediabag.insert("titlepage.png", mimetype, contents)
  return meta
end


This doesn't seem to work. Checking with asserts, the image exists and is 
found by pandoc.mediabag.fetch. Then it is successfully added to the 
mediabag.

But the output epub doesn't have 'titlepage.png' included in the media 
directory.

The only clue that I can find in the documentation is that 'the 
pandoc.mediabag <https://pandoc.org/lua-filters.html#module-pandoc.mediabag>
 module allows access to the “mediabag,” which stores binary content such 
as images that *may* be included in the final document.'

My best guess is that the epub writer checks to see if files in the 
mediabag are being used, and finding no apparent use here, discards 
titlepage.png. But I can't read Haskell, and I haven't seen such checks in 
my attempts to understand at the writer source.

Is it instead something to do with returning an unchanged value for meta in 
the filter?

If so, or if not, does anyone happen to know a good way to manage this?

Thanks!

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