You can write a filedocument to a folder (if the server has rights there) with a javaaction (param filedoc FileDocument, param fullpath String)
InputStream in = Core.getFileDocumentContent(getContext(), filedoc.getMendixObject());
OutputStream out = new FileOutputStream(fullpath);
org.apache.commons.io.IOUtils.copy(in, out);
in.close();
out.close();