I need to write a workflow that reads the content of attachment of a list item, convert it to base64 encoded string and then invoke a custom REST service.
I know I can access "/_api/web/lists/getbytitle('MyList')/items(" + id + ")/AttachmentFiles" and get the server relative URL of the attachment from result.
But I'm not sure how to get the contents of that file and convert it to base64 string.
Update: I've found the following REST API that allows me to read the content of the attachment. "/_api/web/getfilebyserverrelativeurl('server relative URL of attachment from earlier call')/$value
Now the question remains that how to get this content in base64 format?