Add doc and test for set group avatar REST API

doc/api/groups.md

@@ -941,6 +941,19 @@ The `shared_runners_setting` attribute determines whether shared runners are ena
 | `disabled_with_override`       | Disables shared runners for all projects and subgroups in this group, but allows subgroups to override this setting. |
 | `disabled_and_unoverridable`   | Disables shared runners for all projects and subgroups in this group, and prevents subgroups from overriding this setting. |
 
+### Upload a group avatar
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/36681) in GitLab 12.9.
+
+To upload an avatar file from your file system, use the `--form` argument. This causes
+curl to post data using the header `Content-Type: multipart/form-data`. The
+`file=` parameter must point to a file on your file system and be preceded by
+`@`. For example:
+
+```shell
+curl --request PUT --header "PRIVATE-TOKEN: <your_access_token>" "https://gitlab.example.com/api/v4/groups/22" --form "avatar=@/tmp/example.png"
+```
+
 ## Remove group
 
 Only available to group owners and administrators.

spec/requests/api/groups_spec.rb

@@ -54,7 +54,7 @@ RSpec.describe API::Groups do
         it_behaves_like 'invalid file upload request'
       end
 
-      context 'when file format is not supported' do
+      context 'when file is too large' do
         let(:file_path) { 'spec/fixtures/big-image.png' }
         let(:message)   { 'is too big' }
 
@@ -661,6 +661,7 @@ RSpec.describe API::Groups do
 
   describe 'PUT /groups/:id' do
     let(:new_group_name) { 'New Group'}
+    let(:file_path) { 'spec/fixtures/dk.png' }
 
     it_behaves_like 'group avatar upload' do
       def make_upload_request
@@ -678,7 +679,8 @@ RSpec.describe API::Groups do
           request_access_enabled: true,
           project_creation_level: "noone",
           subgroup_creation_level: "maintainer",
-          default_branch_protection: ::Gitlab::Access::MAINTAINER_PROJECT_ACCESS
+          default_branch_protection: ::Gitlab::Access::MAINTAINER_PROJECT_ACCESS,
+          avatar: fixture_file_upload(file_path)
         }
 
         expect(response).to have_gitlab_http_status(:ok)
@@ -701,6 +703,7 @@ RSpec.describe API::Groups do
         expect(json_response['shared_projects']).to be_an Array
         expect(json_response['shared_projects'].length).to eq(0)
         expect(json_response['default_branch_protection']).to eq(::Gitlab::Access::MAINTAINER_PROJECT_ACCESS)
+        expect(json_response['avatar_url']).to end_with('dk.png')
       end
 
       context 'updating the `default_branch_protection` attribute' do