added dynamic thumbnails var

roles/custom/matrix-dendrite/defaults/main.yml

@@ -211,3 +211,7 @@ matrix_dendrite_userapi_auto_join_rooms: []
 # statistics reporting configuration. These statistics contain the server
 # name, number of active users and some information on your deployment config.
 matrix_dendrite_report_stats: false
+
+# dynamically generate thumbnails for media content
+matrix_dendrite_mediaapi_dynamic_thumbnails: false
+matrix_dendrite_mediaapi_max_thumbnail_generators: 10

roles/custom/matrix-dendrite/templates/dendrite/dendrite.yaml.j2

@@ -293,10 +293,10 @@ media_api:
   max_file_size_bytes: {{ matrix_dendrite_max_file_size_bytes|to_json }}
 
   # Whether to dynamically generate thumbnails if needed.
-  dynamic_thumbnails: false
+  dynamic_thumbnails: {{ matrix_dendrite_mediaapi_dynamic_thumbnails }}
 
   # The maximum number of simultaneous thumbnail generators to run.
-  max_thumbnail_generators: 10
+  max_thumbnail_generators: {{ matrix_dendrite_mediaapi_max_thumbnails_generators }}
 
   # A list of thumbnail sizes to be generated for media content.
   thumbnail_sizes: