We've found that messages with RTF formatting that contain embedded images
contain attachments without a "__properties_version1.0" stream.
As the current code is built around the "__properties_version1.0" stream,
these are skipped for now.
These image attachments do contain streams named "Ole" and "MailStream"
that should help with decoding/parsing in the future, but that's a bigger
project.
The fourth argument to `re.sub` is `count`, but `re.I` (a flag) was passed
instead.
Because if this, messages with a lower-case "content-type" header would
never have their content-type header removed, leading to parse errors.
By explicity naming the parameter (`flags=`) to re.sub, the match
actually becomes case-insensitive.
