RexE's questions

If people's computers can decide what to do with files based on their extensions (.pdf, .mp3, .docx, .dotx, etc.), why do web servers and email apps need to also specify a MIME type?

I can see this being useful if you want to deliver a file in a way that differs from the default (e.g. show a .py or .html file as plain text) or if the file extension is unregistered on the client computer, but in most cases, if I'm creating a new file format (say .foobaz) and want to enable people to share these files on the web and over email, shouldn't it be sufficient to rely on the client recognizing the file extension?

My parents' domain is registered with Google Apps (standard edition). Currently, the only service they are using is email (which they access via IMAP in Outlook). I'd like to register this same domain with a hosted Exchange service so that they can sync their Exchange calendar, tasks, and contacts with Outlook as well. I'd like to keep their email on Google for a couple of reasons. Is this possible?

I would like to access a folder of mp3 files on my local Windows machine through http:// addresses. For example, typing http://localhost:9999/songs/test.mp3 into my browser would play test.mp3, which sits in a specified folder on my C: drive. What is the very simplest way to do this?

(Background: a program I'm using wants me to enter the URLs of these files, but assumes they are remote and accessed over http. It doesn't accept URLs of the form file://C/Users.... So, I'd like to give these local files addresses that makes them "look" remote.)

I have heard many times that HTTPS should be used for transferring private data, since HTTP is vulnerable to eavesdroppers. But in practical terms, just who is capable of eavesdropping on a given surfer's HTTP traffic? Their ISP? Other people on the same LAN? Anyone who knows their IP address?

I know that WPA/WPA2 encryption is a pretty good safeguard against snooping by people outside a wireless network. But can someone in the network (i.e. who has been given the shared key) monitor/intercept the traffic (e.g. plain http browsing) of another user on the same network?

EDIT: additional scenario: Alice is on wireless; Bob is connected via ethernet cable to the router. Can Alice intercept Bob's traffic?