Make regex work with multiple links

Modify specs to use public method instead

app/models/project_services/chat_message/base_message.rb

@@ -4,6 +4,8 @@ require 'slack-notifier'
 
 module ChatMessage
   class BaseMessage
+    RELATIVE_LINK_REGEX = /!\[[^\]]*\]\((\/uploads\/[^\)]*)\)/.freeze
+
     attr_reader :markdown
     attr_reader :user_full_name
     attr_reader :user_name
@@ -59,7 +61,11 @@ module ChatMessage
     end
 
     def format(string)
-      Slack::Notifier::LinkFormatter.format(string)
+      Slack::Notifier::LinkFormatter.format(format_relative_links(string))
+    end
+
+    def format_relative_links(string)
+      string.gsub(RELATIVE_LINK_REGEX, "#{project_url}\\1")
     end
 
     def attachment_color

changelogs/unreleased/36716-issue-reference-changes-when-adding-to-epic.yml

+---
+title: Fix relative links in Slack message
+merge_request: 22608
+author:
+type: fixed

spec/models/project_services/chat_message/base_message_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe ChatMessage::BaseMessage do
+  let(:base_message) { described_class.new(args) }
+  let(:args) { { project_url: 'https://gitlab-domain.com' } }
+
+  describe '#fallback' do
+    subject { base_message.fallback }
+
+    before do
+      allow(base_message).to receive(:message).and_return(message)
+    end
+
+    context 'without relative links' do
+      let(:message) { 'Just another *markdown* message' }
+
+      it { is_expected.to eq(message) }
+    end
+
+    context 'with relative links' do
+      let(:message) { 'Check this out ![Screenshot1](/uploads/Screenshot1.png)' }
+
+      it { is_expected.to eq('Check this out https://gitlab-domain.com/uploads/Screenshot1.png') }
+    end
+
+    context 'with multiple relative links' do
+      let(:message) { 'Check this out ![Screenshot1](/uploads/Screenshot1.png). And this ![Screenshot2](/uploads/Screenshot2.png)' }
+
+      it { is_expected.to eq('Check this out https://gitlab-domain.com/uploads/Screenshot1.png. And this https://gitlab-domain.com/uploads/Screenshot2.png') }
+    end
+  end
+end