Set default content-type to 'text/html'

[DoWhileGeek]

The Mime library defaults to 'application/octet-stream', but for folks
who want extentionless urls served by s3, you have to take the
extention off the filename.

Resolves #7.

[jamestalmage]

I would rather set this as a config parameter instead of making this assumption automatically.

I would accept a PR that did the following things:

• export mime.define so users on this module so users can add custom mime types.
• reliably detect when mime can't find a type and return config.defaultMimeType
• adds a mimeType parameter to aws-upload.conf.js. This must be a function that takes a absolute file path as it's only argument and returns a mime type string or null. If null is returned, the default mime type lookup will be used.
• adds unit tests for both the above.
• adds documentation for both the above.

[DoWhileGeek]

@jamestalmage I did some testing with the mime library to test the mapping you suggested, and empty strings are not mapped the way you are wanting.

> mime.define({"text/html": ["", "html"]})
undefined
> mime.lookup("index")
'application/octet-stream'
> mime.lookup("")
'text/html'

Not to mention, this mapping is a little out of scope on what I wanted to help with. How do you feel instead about defaultMimeType that takes in a string that mime defaults to if it cant find anything?

[jamestalmage]

I was simply suggesting we write our own implementation in from of the mime library. I didn't even realize mime.define existed. That changes things a bit. See the updated description.

How would we decide that mime "couldn't find anything"? It returns application/octet-stream by default, but what if someone did this:

mime.define('application/octest-stream': ['my-custom-ext'])

Then anything with that custom extension will also return application/octest-stream, but that doesn't mean we don't know what it did.

I'd probably wrap in a try / finally:

var originalDefaultType = mime.default_type
try {
   mime.default_type = null; // or some reasonably unique string if setting to null causes problems.
   var mimeType =  mime(ext);
   return mimeType || conf.defaultMimeType;
} finally {
  mime.default_type = originalDefaultType;
}