This option enables you to specify the URL from where all URLs will be relative to this option is only used when the relative_urls option is set to true. The value of this option is set to the current document by default.
Remember this option doesn't do much when using the MCFileManager as a TinyMCE plugin since TinyMCE does the URL convertion, consult the TinyMCE manuals for details on URL convertion.
Example of usage in init call on page level:
mcFileManager.init({
document_base_url : "/somedir/somedoc.html"
});
Example of usage in mcFileManager.open call
<a href="javascript:mcFileManager.open('example1','url','','',{document_base_url : "/somedir/somedoc.html"});">[Browse]</a>