How can I integrate file sharing and storage in Ning?

So far the inability to effectively store files in Ning has been a large limitation to its effectiveness in the online professional learning community (ePLC) I want to create for Maths teachers. In the community I hope the e-space becomes a place for discussion, collaborating and sharing resources and links. How to do the latter two?

Ning does Social very well, but it also does wiki poorly (compared to dedicated wikis) and file storage basically unsupported. Grou.ps at least has a file storage page, but even that cannot be heirachically categorized (It does have tagging with tag clouds, however it seems that you can only search for one tag at a time (unlike delicious) which is limiting if a large number of files are stored).

The challenge is how to have a system in which these files can easily be uploaded, accessed, browsed and searched ... and integrated with Ning. Integration is a problem since using an external shared hosted site means that the members will also have to be added to the shared file site, which is problematic and time consuming with constantly joining new members. The site can be embedded, however I believe this can be problematic for some browsers and I also have not had any luck embedding spreadsheets in Ning. I think box.net does embed, but I often get the request to sign-in to box.net when I open that page on Ning. The other option is to use a tab as a link to an external file repository site.

Ideally I would like:
- heirarchical browsing (I still believe in the value of heirachical categorisation (it is almost inbuilt in our thinking and is also how computer data is stored and accessed such as windows explorer, and so familiar to almost all users), though tag integration would be a bonus. ) (e.g. RESOURCES > Algebra > Linear Algebra
- descriptions after each file name (very handy so that the user does not have to open the file to see if they want to open or download it).
- ability to search files
- nice to have a voting/popularity rating so that the user can list by most popular
- the storage of quality (not quantity) resources, to make it easy+quick to get what you want; which is essential for busy teachers and particularly the novice/low frequency/disinclined web users.

Does anyone know of a hosted file repository, wiki, or other tool that has all of these features, or one they would recommend?

Some extra thoughts:

- Wiki where files are stored at the bottom of a page and links made to them in the page.
Pros: can write in descriptions, can search using wiki search feature
Cons: No "list by" feature such as "list by most popular"; two step process for adding resources so not user friendly; I also believe that large number of files can slow wiki down due to flat structure.

- Hosted file storage.
Pros: Many allow categorisation, easy to upload and download
Cons: Most do not have the ability to write file descriptions; no most popular listing; Some only have one layer of categorisation)

- Existing open access teaching resource sites. e.g. Curriki
Pros: huge volume of resources already existing
Cons: community created content becomes 'lost'; also not a shared site where some users may be disinclined to upload resources (not as 'safe' an environment as the community Ning); inability to vet the quality of the content.