With the advanced, utilities workflow component, From variable to payload you can overwrite the current payload of the workflow with the contents of a workflow variable. The parameters are:
Variable name
The name of the variable to copy the data from.
Decode base64
If the data in the variable is stored as an encoded base64 data stream, then you can enable this to convert the base64 data stream into a binary data stream.
Input type
The input type of the new, changed payload.
