fix typo
add methods for creating Attachment annotations and appending to them
add support for attachment annotations (in addition to oembed)
deleteAssociatedFiles param now respects attachments annotations

this handles when multiple attachments annotations exist, too, e.g.

attachmentAnnotation1 {
file1
file2
}
attachmentAnnotation2 {
file3
file4
}
In this case, all 4 files get deleted