title: Pandoc cannot find image path referenced in external HTML file pulled into R Markdown, despite using here() package
body: <p>I am producing an HTML document through R Markdown, which uses a few external HTML documents for headers and footers. I am also using the here() package to allow relative paths throughout my project.
  However, when using YAML to include the external HTML through R code, the relative paths within those files do not work.</p>
  <p>This is the YAML header of the R Markdown code:</p>
  <pre><code>---
  title: &quot;Test&quot;
  output:
    html_document:
      includes:
        in_header: !expr &quot;here::here('FORMATTING','favicon.html')&quot;
  ---
  </code></pre>
  <p>The file 'favicon.html' has the following code:</p>
  <pre><code>&lt;link rel=&quot;icon&quot; type=&quot;image/png&quot; href=&quot;favicon.ico&quot;/&gt;
  </code></pre>
  <p>My folder structure is:</p>
  <pre><code>Main
  │   └──.here
  ├──SCRIPTS
  │   └──test_code.Rmd
  ├──FORMATTING
  │   └──favicon.html
  │   └──favicon.ico
  </code></pre>
  <p>When I run the script using 'Knit' button in RStudio, I get the following error:</p>
  <pre><code>File favicon.ico not found in resource path
  Error: pandoc document conversion failed with error 99
  </code></pre>
  <p>It seems that pandoc can find the HTML files just fine using there R code, but then it seems to want an absolute file path for the actual image.</p>
  <p>How can I ensure pandoc finds the images without having to resort to absolute paths?</p>