Fix base64-encoded file uploads not being possible (#12748)

Fix #3804, Fix #5776

spec/support/examples/models/concerns/account_header.rb

@@ -0,0 +1,23 @@
+# frozen_string_literal: true
+
+shared_examples 'AccountHeader' do |fabricator|
+  describe 'base64-encoded files' do
+    let(:base64_attachment) { "data:image/jpeg;base64,#{Base64.encode64(attachment_fixture('attachment.jpg').read)}" }
+    let(:account) { Fabricate(fabricator, header: base64_attachment) }
+
+    it 'saves header' do
+      expect(account.persisted?).to be true
+      expect(account.header).to_not be_nil
+    end
+
+    it 'gives the header a file name' do
+      expect(account.header_file_name).to_not be_blank
+    end
+
+    it 'saves a new header under a different file name' do
+      previous_file_name = account.header_file_name
+      account.update(header: base64_attachment)
+      expect(account.header_file_name).to_not eq previous_file_name
+    end
+  end
+end