Add file extension to images

image-extension.lua

local known_exts = pandoc.List{".png", ".jpg", "jpeg"}
if FORMAT:match 'tex' then
  known_exts = pandoc.List {
    ".pdf", ".png", ".jpg", ".mps", ".jpeg", ".jbig2", ".jb2"
  }
elseif FORMAT:match 'html' then
  known_exts = pandoc.List {
    ".svg", ".png", ".jpg", ".jpeg", ".webp", ".gif"
  }
end

known_exts:extend(known_exts:map(string.upper))

local function add_extension(basename, ext)
  return basename .. (ext:sub(1,1) == '.' and ext or '.' .. ext)
end

local function file_exists(basename)
  return function (ext)
    local f = io.open(add_extension(basename, ext), 'r')
    return f and io.close(f)
  end
end

function Image (img)
  -- don't do anything if the image already has an extension
  if img.src:match '(%..+)$' then return end

  -- if the default extension has been given, use that; otherwise
  -- try all image extensions and use the first for which an image
  -- exists.
  img.src = add_extension(
    img.src,
    PANDOC_READER_OPTIONS.default_image_extension ~= ''
      and PANDOC_READER_OPTIONS.default_image_extension
      or (known_exts:find_if(file_exists(img.src)) or '')
  )
  return img
end