MM-17438: allow attaching nouser files to posts (#11837)

* tweak AttachToPost formatting

* test attaching files uploaded by nouser

* MM-17438: allow attaching nouser files to posts

app/plugin_api_test.go

@@ -319,6 +319,7 @@ func TestPluginAPIGetFile(t *testing.T) {
 	require.NotNil(t, err)
 	require.Nil(t, data)
 }
+
 func TestPluginAPISavePluginConfig(t *testing.T) {
 	th := Setup(t).InitBasic()
 	defer th.TearDown()
@@ -1244,3 +1245,37 @@ func TestPluginCreateBot(t *testing.T) {
 	require.NotNil(t, err)
 
 }
+
+func TestPluginCreatePostWithUploadedFile(t *testing.T) {
+	th := Setup(t).InitBasic()
+	defer th.TearDown()
+	api := th.SetupPluginAPI()
+
+	data := []byte("Hello World")
+	channelId := th.BasicChannel.Id
+	filename := "testGetFile"
+	fileInfo, err := api.UploadFile(data, channelId, filename)
+	require.Nil(t, err)
+	defer func() {
+		th.App.Srv.Store.FileInfo().PermanentDelete(fileInfo.Id)
+		th.App.RemoveFile(fileInfo.Path)
+	}()
+
+	actualData, err := api.GetFile(fileInfo.Id)
+	require.Nil(t, err)
+	assert.Equal(t, data, actualData)
+
+	userId := th.BasicUser.Id
+	post, err := api.CreatePost(&model.Post{
+		Message:   "test",
+		UserId:    userId,
+		ChannelId: channelId,
+		FileIds:   model.StringArray{fileInfo.Id},
+	})
+	require.Nil(t, err)
+	assert.Equal(t, model.StringArray{fileInfo.Id}, post.FileIds)
+
+	actualPost, err := api.GetPost(post.Id)
+	require.Nil(t, err)
+	assert.Equal(t, model.StringArray{fileInfo.Id}, actualPost.FileIds)
+}