Add GetFile API in app and plugin (#9820)

app/plugin_api.go

@@ -484,6 +484,10 @@ func (api *PluginAPI) ReadFile(path string) ([]byte, *model.AppError) {
 	return api.app.ReadFile(path)
 }
 
+func (api *PluginAPI) GetFile(fileId string) ([]byte, *model.AppError) {
+	return api.app.GetFile(fileId)
+}
+
 func (api *PluginAPI) UploadFile(data []byte, channelId string, filename string) (*model.FileInfo, *model.AppError) {
 	return api.app.UploadFile(data, channelId, filename)
 }