Convert document to base64 in response body for Web API Object (External client wants to call Appian Web API to retrieve documents)

This is the code from the pattern "Download Document" I selected when creating a new API. But this does only return the file. I am not aware that you can return a JSON structure that includes the base64 encoded file.

EDIT: But you could include a number of additional parameters as headers.

a!localVariables(
  local!pathArray: fn!cast('type!{http://www.appian.com/ae/types/2009}Text?list', http!request.pathSegments),
  local!document: tointeger(index(local!pathArray, 1, null)),
  /* We don't want to serve arbitrary documents because it could be a security risk */
  local!extensionWhitelist: {"pdf", "txt", "png", "jpg", "jpeg"},
  if(
    /*
    * The path must be only 1 value, the document ID, and that ID must be a number.
    * Otherwise, we return a 404 Not Found
    */
    or(
      length(local!pathArray) <> 1,
      isnull(local!document),
      not(contains(local!extensionWhitelist, document(local!document, "extension")))
    ),
    a!httpResponse(
      statusCode: 404,
      body: "404 Not Found",
      headers: {}
    ),
    a!httpResponse(
      /*
      * If the query parameter "attachment" is set to true,
      * set an HTTP header that tells the client that the body of the response
      * should be downloaded. This overrides the default content-disposition
      * of "inline; filename=.". We will automatically
      * set the content-type and length.
      */
      headers: if(
        http!request.queryParameters.attachment,
        a!httpHeader(
          name: "Content-Disposition",
          value: concat(
            "attachment; filename=""",
            document(local!document, "name"),
            ".",
            document(local!document, "extension"),
            """"
          )
        ),
        {}
      ),
      body: todocument(local!document)
    )
  )
)

Perhaps something you can achieve by using this plugin?

 [mention:e7b082a6b98049899e3541011e57f30e:f7d226abd59f475c9d224a79e3f0ec07]