[s3boto] set the correct Content-Type for file

[robinelvin]

As with #303 / #320 the mime type and therefore Content-Type for files uploaded using the S3 handler defaults to application/octet-stream when the type cannot be guessed. Looking at the code it appears to me that content_type is guessed from the file extension.
I could be wrong here but from what I can gather this is what happens:
Passing in a content type to the _save() method of S3BotoStorage appears to be used but is not used when it reaches the write() method in S3BotoStorageFile where only the file extension is considered. My experience as a consumer of the storage backend is consistent with this observation.

[jschneier]

Fixed a while ago.