default to success and hide indicators if not needed

packages/stream_chat/lib/src/models/attachment.dart

@@ -58,7 +58,7 @@ class Attachment {
   final AttachmentFile file;
 
   /// The current upload state of the attachment
-  final UploadState uploadState;
+  UploadState uploadState;
 
   /// Map of custom channel extraData
   @JsonKey(includeIfNull: false)
@@ -123,10 +123,15 @@ class Attachment {
     this.actions,
     this.extraData,
     this.file,
-    this.uploadState,
+    UploadState uploadState,
   })  : id = id ?? Uuid().v4(),
         title = title ?? file?.name,
-        localUri = file?.path != null ? Uri.parse(file.path) : null;
+        localUri = file?.path != null ? Uri.parse(file.path) : null {
+    this.uploadState = uploadState ??
+        ((assetUrl != null || imageUrl != null)
+            ? UploadState.success()
+            : UploadState.preparing());
+  }
 
   /// Create a new instance from a json
   factory Attachment.fromJson(Map<String, dynamic> json) {