Is there a configuration in the smart service node(s), where I could change the

AFAIK you would need a custom expression or smart service to directly change the file extension of an existing file. However, depending on your requirements it may help to create the file with the target extension to start with, using the Text from Template plugin:

forum.appian.com/.../summary

You can create a document with target extension as suggested by Tom.

If this does not work. One solution I can think of is to use "Appian Document from Base64 String" custom smart service to change extension of a file. For this, you will need to generate the base64 representation of the source document and pass it as input to smart service along with new extension and other file parameters.

Place this smart service in an independent sub process and use this subprocess for renaming document. Set data management policy to delete the subprocess as soon as its completed to free up the memory. Avoid this approach for file size > 10 MB, for such cases better would be to find/develop a custom plugin to change the document extension.

Thank you Ryan and Akhilesh and I have achieved by using "Appian Document from Base64 String" smart service