Here is v1.00 of the Extended Download Plugin installer.
This allows any file type to be downloaded to any mounted writable partition. It's nly working on CFW for now as this installer includes a patched download_plugin.sprx from Rebug 4.84.2 REX.
NOTE: it should work on all recent CFW, but it has only been tested on the latest Rebug. There might be bugs, so use at your own risk.
Known bug: Background downloading does not work fully for unknown file types, They will download fully, but they will stay in dev_hdd0/vsh/task/xxxxxxxx/ , they will also say 99% on the XMB even though they are actually 100%.
This has extra paths added to dev_hdd0, and a new media section which allows direct downloads into dev_hdd0/photo, dev_hdd0/video and dev_hdd0/music. Now all items now have custom icons and I added warning symbols to dev_blind paths.
Thanks to kozarovv for helping me with research and testing, and to Berion for the icons.
Images of v1.00: