Base64 is returning null in scoped application REST API explorer

[APRAJITAPAL]

I have scoped application. On Request I am attaching pdf file. While using GET https method in scripted REST API, I am getting null in Base64. However I am getting file name, content type, sys_id. Using below code-

_readAttachmentAsBase64: function(attachmentSysId) {

        try {

            /*var ais = new GlideSysAttachmentInputStream(String(attachmentSysId));

            var baos = new Packages.java.io.ByteArrayOutputStream();

            ais.writeTo(baos);

            var bytes = baos.toByteArray();

            baos.close();

            return this._b64Encode(bytes);

        } catch (e) {

            gs.error('GAD ERR: base64 read failed for attachment ' + attachmentSysId + ': ' + e.message);

            return null;

        }*/

        var gsa = new GlideSysAttachment();

        var inputStream = gsa.getContentStream(attachmentSysId);

        if(!inputStream){

            gs.error("No stream for attachment:"+attachmentSysId)

    return null;

        }

        var baos = new Packages.java.io.ByteArrayOutputStream();

        var buffer = Packages.java.lang.reflect.Array.newInstance(java.lang.Byte.TYPE, 4096);

        var bytesRead;

        while((bytesRead = inputStream.read(buffer)) != -1){

            baos.write(buffer,0,bytesRead);

        }var bytes = baos.toByteArray();

        var base64 = GlideStringUtil.base64Encode(bytes);

        return base64;

        } catch (e){

            gs.error("STREAM ERROR:"+e.message);

            return null;

        }

    },