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=<filename>.<extension>". 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)
    )
  )
)

Thanks Stefan.  I think on our side the confusion was whether the resulting URL from that Web API object could be used to fetch the document as part of a backend process (and not have to convert the document to base64 string in the response body as a response).

E.g., whether the client could do the following Python call and use the resulting document in their process:

Our external client has informed us that they should be able to use that URL to fetch the document, so I think we should be able to proceed.

Perhaps something you can achieve by using this plugin?

 Base64 Document Database Integration