I have been able to upload files using the following plain HTML:
<html>
<body>
<form action="
https://docs.google.com/feeds/default/media/document
%3A<document id>" method="POST">
<input type="hidden" name="xdp:alt" value="json-xd" />
<input type="hidden" name="xdh:Authorization" value="AuthSub
token=<token>" />
<input type="hidden" name="xdh:GData-Version" value="3.0" />
<input type="hidden" name="xdh:Content-Type" value="text/plain" />
<input type="hidden" name="xdh:If-Match" value="<etag or * if you
don't care>" />
<input type="hidden" name="xdh:X-HTTP-Method-Override" value="PUT" />
<textarea name="xdp:body">Whatever value you want here!</textarea>
<input type="submit" value="update" />
</form>
Replace <token> with your AuthSub token and provide a value for the
etag, such as "*". You can dynamically build the form with JS and
submit.
Bobby