This is a bad idea to fork Nuxeo code and Nuxeo provide a framework dedicated for developer to help them to plug their own code on the platform. This is provided by our extension point system. I let you discover this by reading this documentation: http://doc.nuxeo.com/x/DIAO

For your case, you want - if I'm right - to create your own logic of document creation from a post.

This is typically what the File Manager Service you need. My piece of advice is to use Nuxeo IDE to create an empty bundle: http://doc.nuxeo.com/x/aoKE

And contribute your File Manager by adding this xml file that will declare your class:

  <?xml version="1.0"?>
  <component name="com.feiwang.filemanager.contribution">

    <require>org.nuxeo.ecm.platform.filemanager.service.FileManagerService</require>

    <extension
      target="org.nuxeo.ecm.platform.filemanager.service.FileManagerService"
      point="plugins">

      <plugin name="yourFileManager"
          class="com.feiwang.filemanager.contribution.YourClass"
          order="0">
        <filter>.*</filter>
      </plugin>
    </extension>
  </component>

Your class must implement the FileImporter interface and you can inherit of AbstractFileImporter.

Think to add into your pom.xml this dependency:

<groupId>org.nuxeo.ecm.platform</groupId>
<artifactId>nuxeo-platform-filemanager-core</artifactId>

Hi you may be more interested with our Flex connector add-on that expose API needed for Flex application.

http://hg.nuxeo.org/addons/nuxeo-platform-flex/

To get source, as usual (until we move the code to github :) :

hg clone http://hg.nuxeo.org/addons/nuxeo-platform-flex/

But, you also may find your need here.