I would like to convert a domino document field of Data Type: MIME Part
into a Data Type: Rich Text
in backend with SSJS
You may be able to do this as part of the usually-undesirable side effect of automatic MIME-to-CD conversion in the API. For example, code like this will turn the Body field of the first doc in the DB from MIME to composite data:
boolean convertMime = session.isConvertMime();
session.setConvertMime(true);
Document doc = database.getAllDocuments().getFirstDocument();
RichTextItem rtitem = (RichTextItem)doc.getFirstItem("Body");
rtitem.compact();
doc.save();
session.setConvertMime(convertMime);
By making sure that the session is converting MIME (which is true
by default, but it's best to maintain any previously-existing value) and then interacting with the MIME_PART item, it will mangle it into CD for you.