Return the full path to an upload in api response

To fetch an upload, a GET request needs to be made to
/namespace_path/project_path/:upload_path. This change includes
this url in the data returned after making an upload.

changelogs/unreleased/25838-include-full-upload-url-in-api-response.yml

+---
+title: Include full path to an upload in api response
+merge_request: 23500
+author: briankabiro
+type: other

doc/api/projects.md

@@ -1836,11 +1836,12 @@ Returned object:
 {
   "alt": "dk",
   "url": "/uploads/66dbcd21ec5d24ed6ea225176098d52b/dk.png",
+  "full_path": "/namespace1/project1/uploads/66dbcd21ec5d24ed6ea225176098d52b/dk.png",
   "markdown": "![dk](/uploads/66dbcd21ec5d24ed6ea225176098d52b/dk.png)"
 }
 ```
 
->**Note**: The returned `url` is relative to the project path.
+>**Note**: The returned `url` is relative to the project path. The returned `full_path` is the absolute path to the file.
 In Markdown contexts, the link is automatically expanded when the format in
 `markdown` is used.
 

lib/api/entities/project_upload.rb

+# frozen_string_literal: true
+
+module API
+  module Entities
+    class ProjectUpload < Grape::Entity
+      include Gitlab::Routing
+
+      expose :markdown_name, as: :alt
+      expose :secure_url, as: :url
+      expose :full_path do |uploader|
+        show_project_uploads_path(
+          uploader.model,
+          uploader.secret,
+          uploader.filename
+        )
+      end
+
+      expose :markdown_link, as: :markdown
+    end
+  end
+end

lib/api/projects.rb

@@ -494,7 +494,9 @@ module API
         requires :file, type: File, desc: 'The file to be uploaded' # rubocop:disable Scalability/FileUploads
       end
       post ":id/uploads" do
-        UploadService.new(user_project, params[:file]).execute.to_h
+        upload = UploadService.new(user_project, params[:file]).execute
+
+        present upload, with: Entities::ProjectUpload
       end
 
       desc 'Get the users list of a project' do

spec/requests/api/projects_spec.rb

@@ -1250,6 +1250,8 @@ describe API::Projects do
       expect(json_response['alt']).to eq("dk")
       expect(json_response['url']).to start_with("/uploads/")
       expect(json_response['url']).to end_with("/dk.png")
+
+      expect(json_response['full_path']).to start_with("/#{project.namespace.path}/#{project.path}/uploads")
     end
   end