You will find the correct syntax in comment https://www.docuware.com/comment/5205#comment-5205
the correct syntax for your xml file for CheckInFromFileSystem is like this:
<?xml version="1.0" encoding="utf-8"?>
<?xml-stylesheet type="text/xsl" href="/DocuWare/Platform/Content/standard.xslt"?>
<CheckInActionParameters Comments="test comment" xmlns:s="http://dev.docuware.com/schema/public/services" xmlns="http://dev.docuware.com/schema/public/services/platform">
The String for CheckInReturnDocument could be "CheckedIn" or "Original".
In Comments attribute you can save a comment to your checked in new file.
Be sure you have post a CheckoutDocument to that document before you post a new checkin . Otherwise you get an error like this:
"Operation is not allowed. The document with ID xxx must be in the state 'In Progress'."
I hope my answer is helpful.