Adding extension for attachments

for the Write Template section[0]
- ​replace instances of content-type with contentType
- replace "compile" with "complies"
- normalize the use of "NOTE:....." in the Attachment Property section vs.
inline text in the ContentType section. I like the NOTE format, but it's up
to you.

If you want to talk about body parts, I think you need to identify them in
the Example.

for the Multipart Requests section[1]
- normalize references to data object, template object, & template data
object.
- change attachements to attachments

[0]
https://github.com/glennblock/extensions/blob/master/attachment.md#write-template
[1]
https://github.com/glennblock/extensions/blob/master/attachment.md#mulipart-requests