From what I found reading through some stuff, 'tulip' is the term used to describe whatever a whistle-blower uploads. It also has some meta-data including a point of discussion, so that the Global Leaks provider (the recipient of information) can ask the sender (the whistle-blower) to upload some more material or explain something. It seems tulips can be set, by the sender, to expire.
Here's a link to the tulip controller - you can mostly figure out what it does reading through the well-structured code:
Perhaps I am hopeless behind the times, but what is a tulip?