Add logging for file upload

Add logging when a file is uploaded to S3 for a file upload question,
or when validation fails due to a file being too big.

This information can help us analyse what file types people are
attempting to or successfully uploading and do analysis of the file
sizes.

app/models/question/file.rb

@@ -22,6 +22,10 @@ def before_save
       tempfile = file.tempfile
       key = file_upload_s3_key(tempfile)
       upload_to_s3(tempfile, key)
+      Rails.logger.info("Uploaded file to S3 for file upload question", {
+        file_size_in_bytes: file.size,
+        file_type: file.content_type,
+      })
 
       self.original_filename = file.original_filename
       self.uploaded_file_key = key
@@ -42,6 +46,10 @@ def file_from_s3
 
     def validate_file_size
       if file.present? && file.size > FILE_UPLOAD_MAX_SIZE_IN_MB.megabytes
+        Rails.logger.info("File upload question validation failed: file too big", {
+          file_size_in_bytes: file.size,
+          file_type: file.content_type,
+        })
         errors.add(:file, :too_big)
       end
     end

spec/models/question/file_spec.rb

@@ -26,6 +26,8 @@
       let(:bucket) { "an-s3-bucket" }
       let(:key) { Faker::Alphanumeric.alphanumeric }
       let(:tempfile) { Tempfile.new(%w[temp-file .png]) }
+      let(:file_size_in_bytes) { 2.megabytes }
+      let(:file_type) { "image/png" }
 
       after do
         tempfile.unlink
@@ -36,10 +38,13 @@
         allow(mock_s3_client).to receive(:put_object)
         allow(Settings.aws).to receive(:file_upload_s3_bucket_name).and_return(bucket)
 
-        allow(uploaded_file).to receive_messages(original_filename: original_filename, tempfile: tempfile)
+        allow(uploaded_file).to receive_messages(original_filename: original_filename, tempfile: tempfile,
+                                                 size: file_size_in_bytes, content_type: file_type)
 
         allow(SecureRandom).to receive(:uuid).and_return key
 
+        allow(Rails.logger).to receive(:info).at_least(:once)
+
         question.file = uploaded_file
 
         question.before_save
@@ -60,6 +65,13 @@
       it "sets the original_filename" do
         expect(question.original_filename).to eq original_filename
       end
+
+      it "logs information about the file" do
+        question.validate
+        expect(Rails.logger).to have_received(:info).with("Uploaded file to S3 for file upload question",
+                                                          { file_size_in_bytes:,
+                                                            file_type: })
+      end
     end
 
     context "when no file was selected (question was optional)" do
@@ -133,16 +145,27 @@
 
     context "when the file size is greater than 7MB" do
       let(:uploaded_file) { instance_double(ActionDispatch::Http::UploadedFile) }
+      let(:file_size_in_bytes) { 7.megabytes + 1 }
+      let(:file_type) { "image/png" }
 
       before do
-        allow(uploaded_file).to receive(:size).and_return(7.megabytes + 1)
+        allow(uploaded_file).to receive_messages(size: file_size_in_bytes, content_type: file_type)
         question.file = uploaded_file
+
+        allow(Rails.logger).to receive(:info).at_least(:once)
       end
 
       it "returns an error" do
         expect(question).not_to be_valid
         expect(question.errors[:file]).to include "The selected file must be smaller than 7MB"
       end
+
+      it "logs information about the file" do
+        question.validate
+        expect(Rails.logger).to have_received(:info).with("File upload question validation failed: file too big",
+                                                          { file_size_in_bytes:,
+                                                            file_type: })
+      end
     end
 
     context "when the file is valid" do