How to send Appian document in base 64 format to external system through Appian Integration?

Hi All

 I am calling one web service through Appian Integeration and getting the below error.

I want to send the appian document in base 64 string.

I checked in appian community and found that appian automatically convert document to base 64 string but facing issue.

the request body of web service is in xml format.

Please refer the below screenshots.

In below request i have to send document in content tag and it should be in base 64 string.

the ri!Attachments is document type.

Many Thanks in advance.