added a swipeable media gallery for chat and folder images and videos with server previews, on-demand originals and zoom, and marked document previews in chat so they no longer pass for photos

lib/api/marianumcloud/talk/chat/get_chat_response.dart

@@ -165,7 +165,17 @@ class RichObjectString {
   String? path;
   String? link;
 
-  RichObjectString(this.type, this.id, this.name, this.path, this.link);
+  /// Only set for `file` objects.
+  String? mimetype;
+
+  RichObjectString(
+    this.type,
+    this.id,
+    this.name,
+    this.path,
+    this.link, [
+    this.mimetype,
+  ]);
 
   factory RichObjectString.fromJson(Map<String, dynamic> json) =>
       _$RichObjectStringFromJson(json);