1. Replace output body_without_headers with original_body
2. Use inject with accumulator when mapping over attachments

These changes will make it easier to add additional processing into this
method in the coming commits.

Move Unix-to-Unix encoding [1] into the `MailHandler` as this will allow
us to extract uuencoded attachments attributes without initialising
`FoiAttachment` instances.

[1] https://en.wikipedia.org/wiki/Uuencoding

For attachments which are encoded with Unix-to-Unix encoding we are
unable process some attachments in order to apply masks or censor rules
to them.

This is down to when the attachments are extracted and saved into the
database. It happens outside of our `MailHandler` library which we're
using to retrieve the original unmasked attachment body for processing.

This change fixes this by checking plain text attachments for begin/end
markers and calculating the relevant hexdigest for comparison.

Fixes #7877