FEATURE: Multisite support for S3 image stores (#6689)

* FEATURE: Multisite support for S3 image stores

* Use File.join to concatenate all paths & fix linting on multisite/s3_store_spec.rb

spec/multisite/s3_store_spec.rb

@@ -0,0 +1,42 @@
+require 'rails_helper'
+require 'file_store/s3_store'
+
+RSpec.describe 'Multisite s3 uploads', type: :multisite do
+  let(:conn) { RailsMultisite::ConnectionManagement }
+  let(:uploaded_file) { file_from_fixtures("smallest.png") }
+
+  let(:upload) do
+    Fabricate(:upload, sha1: Digest::SHA1.hexdigest(File.read(uploaded_file)))
+  end
+
+  let(:s3_client) { Aws::S3::Client.new(stub_responses: true) }
+
+  let(:s3_helper) do
+    S3Helper.new(SiteSetting.s3_upload_bucket, '', client: s3_client)
+  end
+
+  let(:store) { FileStore::S3Store.new(s3_helper) }
+
+  context 'uploading to s3' do
+    before(:each) do
+      SiteSetting.s3_upload_bucket = "some-really-cool-bucket"
+      SiteSetting.s3_access_key_id = "s3-access-key-id"
+      SiteSetting.s3_secret_access_key = "s3-secret-access-key"
+      SiteSetting.enable_s3_uploads = true
+    end
+
+    describe "#store_upload" do
+      it "returns the correct url for default and second multisite db" do
+        expect(store.store_upload(uploaded_file, upload)).to eq(
+          "//#{SiteSetting.s3_upload_bucket}.s3.dualstack.us-east-1.amazonaws.com/original/1X/c530c06cf89c410c0355d7852644a73fc3ec8c04.png"
+        )
+
+        conn.with_connection('second') do
+          expect(store.store_upload(uploaded_file, upload)).to eq(
+            "//#{SiteSetting.s3_upload_bucket}.s3.dualstack.us-east-1.amazonaws.com/uploads/second/original/1X/c530c06cf89c410c0355d7852644a73fc3ec8c04.png"
+          )
+        end
+      end
+    end
+  end
+end