They're medical files (CTs and stuff), and the commercial imaging libraries cost huge amounts.
Fortunately, I haven't come across many of these files in practice, but as I'm trying to develop an analysis tool - I don't want to find that I can't import them.
Perhaps, there are some PACS admins here who know how images are stored. At the moment, pretty much everything comes off the scanners as uncompressed bitmaps - but I don't want to find that once they come back off the long-term archive, they're compressed.