How to convert an Appian Document (image) to a Base64 string?

Hi Community,

I'm building an HTML-to-PDF generation flow and need to embed an employee photo (stored as an Appian Document) directly into an <img> tag as a Base64 data URL, like this:

<img src="data:image/png;base64,{BASE64_STRING}" />

I want to construct the value in an Appian expression as:

"data:image/png;base64," & base64Data

My question: Is there a built-in Appian expression function to convert a Document to a Base64 string?

Using the Appian document URL directly in <img src> does not work.

Thank you