Fix rendering of service desk notes

When custom template is used, then default mailer views are not used
for rendering the email, but `markdown` for rendering the email is
called directly. We should make sure that proper banzain pipeline is
set for rendering and also note's author is set.

app/mailers/emails/service_desk.rb

@@ -58,10 +58,12 @@ module Emails
 
     def template_content(email_type)
       template = Gitlab::Template::ServiceDeskTemplate.find(email_type, @project)
-
       text = substitute_template_replacements(template.content)
 
-      markdown(text, project: @project)
+      context = { project: @project, pipeline: :email }
+      context[:author] = @note.author if email_type == 'new_note'
+
+      markdown(text, context)
     rescue Gitlab::Template::Finders::RepoTemplateFinder::FileNotFoundError
       nil
     end

changelogs/unreleased/fix_sdesk_links.yml

+---
+title: Render correct URLs for uploads in service desk issues when custom template
+  is used.
+merge_request: 45772
+author:
+type: fixed

spec/mailers/emails/service_desk_spec.rb

@@ -183,6 +183,25 @@ RSpec.describe Emails::ServiceDesk do
 
         it_behaves_like 'handle template content', 'new_note'
       end
+
+      context 'with upload link in the note' do
+        let_it_be(:upload_path) { '/uploads/e90decf88d8f96fe9e1389afc2e4a91f/test.jpg' }
+        let_it_be(:note) { create(:note_on_issue, noteable: issue, project: project, note: "a new comment with [file](#{upload_path})") }
+
+        let(:template_content) { 'some text %{ NOTE_TEXT  }' }
+        let(:expected_body) { %Q(some text a new comment with <a href="#{project.web_url}#{upload_path}" data-link="true" class="gfm">file</a>) }
+
+        it_behaves_like 'handle template content', 'new_note'
+      end
+
+      context 'with all-user reference in a an external author comment' do
+        let_it_be(:note) { create(:note_on_issue, noteable: issue, project: project, note: "Hey @all, just a ping", author: User.support_bot) }
+
+        let(:template_content) { 'some text %{ NOTE_TEXT  }' }
+        let(:expected_body) { 'Hey , just a ping' }
+
+        it_behaves_like 'handle template content', 'new_note'
+      end
     end
   end
 end