[Excel Analyzer] add spreadsheet received callback

Allow Alaveteli to take action if an email contains an Excel
spreadsheet.

gems/excel_analyzer/lib/excel_analyzer.rb

@@ -8,6 +8,17 @@
 # detect hidden data within spreadsheet attachments in emails. It supports .xls
 # and .xlsx file formats.
 module ExcelAnalyzer
+  # A configurable callable that gets executed when an email with a spreadsheet
+  # attachment is analyzed. This allows for custom handling of the spreadsheet
+  # data.
+  #
+  # @example Set a custom callable to handle received spreadsheets
+  #   ExcelAnalyzer.on_spreadsheet_received = ->(blob) { process(blob) }
+  #
+  # @!attribute [rw] on_spreadsheet_received
+  # @return [Proc] the callable to run for spreadsheet attachments
+  mattr_accessor :on_spreadsheet_received, default: ->(blob) {}
+
   # Provides the list of content types that the ExcelAnalyzer will attempt to
   # analyze in search of hidden data. It currently includes content types for
   # .xls and .xlsx files.

gems/excel_analyzer/lib/excel_analyzer/eml_analyzer.rb

@@ -26,7 +26,11 @@ def metadata
           _1[:content_type]
         end
 
-        return { content_types: }
+        if content_types.any? { ExcelAnalyzer.content_types.include?(_1) }
+          # rubocop:disable Style/RescueModifier
+          ExcelAnalyzer.on_spreadsheet_received.call(blob) rescue nil
+          # rubocop:enable Style/RescueModifier
+        end
       end
 
       {}

gems/excel_analyzer/spec/excel_analyzer/eml_analyzer_spec.rb

@@ -19,8 +19,15 @@
   end
 
   describe "#metadata" do
+    around do |example|
+      original_callback = ExcelAnalyzer.on_spreadsheet_received
+      ExcelAnalyzer.on_spreadsheet_received = ->(blob) {}
+      example.call
+      ExcelAnalyzer.on_spreadsheet_received = original_callback
+    end
+
     let(:mail) do
-      Mail.new { add_file File.join(__dir__, "../fixtures/data.xls") }
+      Mail.new { add_file File.join(__dir__, "../fixtures/plain.txt") }
     end
 
     let(:io) { double(:File, path: "blob/path") }
@@ -30,6 +37,49 @@
 
     before { allow(Mail).to receive(:read).with("blob/path").and_return(mail) }
 
-    it { is_expected.to include(content_types: ["application/vnd.ms-excel"]) }
+    it { is_expected.to eq({}) }
+
+    context "when mail contains XLS attachment" do
+      let(:mail) do
+        Mail.new { add_file File.join(__dir__, "../fixtures/data.xls") }
+      end
+
+      it { is_expected.to eq({}) }
+
+      it "calls on_spreadsheet_received callback" do
+        expect(ExcelAnalyzer.on_spreadsheet_received).
+          to receive(:call).with(blob)
+        metadata
+      end
+    end
+
+    context "when mail contains XLSX attachment" do
+      let(:mail) do
+        Mail.new { add_file File.join(__dir__, "../fixtures/data.xlsx") }
+      end
+
+      it { is_expected.to eq({}) }
+      it "calls on_spreadsheet_received callback" do
+        expect(ExcelAnalyzer.on_spreadsheet_received).
+          to receive(:call).with(blob)
+        metadata
+      end
+    end
+
+    context "when mail contains XLS and XLSX attachment" do
+      let(:mail) do
+        Mail.new do
+          add_file File.join(__dir__, "../fixtures/data.xls")
+          add_file File.join(__dir__, "../fixtures/data.xlsx")
+        end
+      end
+
+      it { is_expected.to eq({}) }
+      it "calls on_spreadsheet_received callback once only" do
+        expect(ExcelAnalyzer.on_spreadsheet_received).
+          to receive(:call).with(blob).once
+        metadata
+      end
+    end
   end
 end