ZIP files get recognized as text/plain

You have to remember that File Framework is a separate project from Bitcache. Bitcache's administration screens are not tailored towards File Framework use cases, but, rather, "raw "data management; Bitcache in itself does not know the file name of any object stored in a Bitcache repository, as by the nature of content-addressable storage (such as Bitcache is based on) a bitstream's hash (also known as a digital fingerprint, i.e. the numbers and letters you refer to) is the one and only identifier for the bitstream. The mapping to file names happens much higher up, in File Framework, and Bitcache knows nothing about it.

Likewise, the MIME types shown by Bitcache are purely for informational purposes, and are guessed on-the-fly using the MIME type autodetection facilities available on your server (typically, Apache's mime.types database as well as the Unix file command). It is quite possible, even probable, that some file types don't get correctly recognized on some servers; that doesn't, however, affect File Framework, which stores a proper MIME type that it detects using more accurate methods than Bitcache has available.

Think of Bitcache as a database utilized by File Framework; in the same way as you wouldn't want to poke around MySQL's innards, nor would you most often need to even care that MySQL exists somewhere beneath your Drupal site, you can safely ignore Bitcache as well as admin/content/bitcache (except if it has bugs ;-))