How to update image using web-service client API in Polarion?

Question

I have a question about the Polarion API. I'm trying to update existing WorkItem in a LiveDoc using webservices api.

Background:
I'm building a script to enable automatic synchronisation between Enterprise Architect and Polarion. The diagrams are created in EA and need to be automatically updated in Polarion. The EA part is done and the diagrams are available in .png format, now only the Polarion part is missing and I have no idea how to start.

1. I would like to upload a diagram.png file to the attachments section.
2. I would like to add the attached diagram.png file to the existing WorkItem description only if status WorkItem is InProgress.

Is there any easy way to do it?

Note: A single synchronisation may involve a larger number of diagrams to be synchronised.

Note: Synchronisation is 1:1, one diagram to one WorkItem.