Another reason--that I am speculating--that Windows Phone 7 Series will not need--at least for most average consumers--a file management system is that there will be a device Search feature--enabled via the search hardware button, which is one of three hardware buttons on the front of the device just below the screen. Moreover, having a file system may seem redundant--again, for the target demographics, which is the average consumer--if the device can categorize and organize files in a logical, natural manner through hubs. Though power users migrating from Windows Mobile may feel cheated, Microsoft's core demographics for Windows Phone 7 is the average consumer so tradeoffs had to be made.
Also, as a consequent to lacking a true file management system, gone will be USB mass storage mode, which was a popular feature for users to plug their phones into a PC and mount their storage card on their desktop to quickly access or migrate data between PCs.
At this point, as far as we know, to get music and media to and from your Windows Phone 7 Series device, you'll need to utilize the Dorado client, which is the Zune/Zune HD synchronization client for the PC, and Office files will be transferred in a method that hasn't been detailed at this time.