To Convert Base64 URL to Base64 Encoded format in WSO2 EI 6.3.0

I am getting email attachment(images) from Gmail API which returns base64url encoded string. But actual encoded is different from this gmail api("/" is replaced by "_" and "+" is replaced by "-" etc). I want to get original base64 encoded string instead of base64 url encoding format. Shall i replace that two symbols("/","+") from gmail api response or can anyone please help me to achieve this? i am trying this using WSO2 EI 6.3.0.

 <call>
                    <endpoint>
                        <http method="get" uri-template="{+uri.var.gmail.apiUrl}/{+uri.var.gmail.apiVersion}/users/{+uri.var.gmail.userId}/messages/{+uri.var.id}/attachments/{+uri.var.attachmentId}"/>
                    </endpoint>
                </call>
 <property description="emailAttachment" expression="//data/text()" name="emailAttachment" scope="default" type="STRING"/>

for eg,

from Gmail API : after that call, it returns following value in "data" key.

_9j_4AAQSkZJRgABAQAASABIAAD_4QBMRXhpZgAATU0AKgAAAAgAAgESAAMAAAABAAEAAIdpAAQAAAABAAAAJgAAAAAAAqACAAQAAAABAAAPwKADAAQAAAABAAAL0AAAAAD_....

original Base64 Encoded value:

/9j/4AAQSkZJRgABAQAASABIAAD/4QBMRXhpZgAATU0AKgAAAAgAAgESAAMAAAABAAEAAIdpAAQAAAABAAAAJgAAAAAAAqACAAQAAAABAAAPwKADAAQAAAABAAAL0AAAAAD/....

Solution

In Javascript the easiest would be to use the replace() method:

newData=data.replace(/-/g, '+').replace(/_/g, '/')