Fix uuencoded attachment processing

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

lib/mail_handler/backends/mail_backend.rb

@@ -424,6 +424,17 @@ def caluclate_hexdigest(body)
           mail, body: mail_body, nested: true
         ) unless mail_body.empty?
 
+        return attributes if attributes
+
+        # check uuencoded attachments which can be located in plain text
+        uuencoded_attributes = all_attributes.inject([]) do |acc, attrs|
+          next acc unless attrs[:content_type] == 'text/plain'
+          acc += uudecode(attrs[:body], attrs[:url_part_number])
+        end
+        attributes ||= uuencoded_attributes.find do |attrs|
+          attrs[:hexdigest] == hexdigest
+        end
+
         attributes
       end
 

spec/lib/mail_handler/backends/mail_backend_spec.rb

@@ -443,12 +443,26 @@
         EML
       ).to_s
 
+      mail_with_uuencoded = Mail.new(
+        <<~EML
+          Subject: Cat
+
+          Hi Cat!
+
+          begin 644 cat.txt
+          #0V%T
+          `
+          end
+        EML
+      ).to_s
+
       Mail.new do
         add_file filename: 'crlf.txt', content: "foo\r\nfoo"
         add_file filename: 'lf.txt', content: "bar\nbar"
         add_file filename: 'crlf-non-ascii.txt', content: "Aberdâr\r\n"
         add_file filename: 'lf-non-ascii.txt', content: "Aberdâr\n"
         add_file filename: 'mail.eml', content: mail_attachment
+        add_file filename: 'uuencoded.eml', content: mail_with_uuencoded
       end
     end
 
@@ -494,6 +508,11 @@
       it { is_expected.to include(body: "bar\nbar") }
     end
 
+    context 'when body has been uuencoded' do
+      let(:body) { "Cat" }
+      it { is_expected.to include(body: "Cat") }
+    end
+
     context 'when body does not match' do
       let(:body) { 'this does not match' }
       it { is_expected.to eq(nil) }