method.
String feedElementId = '0D5D0000000KtW3';
ConnectApi.CommentInput commentInput = new ConnectApi.CommentInput();
ConnectApi.MessageBodyInput messageBodyInput = new ConnectApi.MessageBodyInput();
ConnectApi.TextSegmentInput textSegmentInput = new ConnectApi.TextSegmentInput();
textSegmentInput.text = 'Enjoy this new file.';
messageBodyInput.messageSegments = new List<ConnectApi.MessageSegmentInput>();
messageBodyInput.messageSegments.add(textSegmentInput);
commentInput.body = messageBodyInput;
ConnectApi.CommentCapabilitiesInput commentCapabilitiesInput = new ConnectApi.CommentCapabilitiesInput();
ConnectApi.ContentCapabilityInput contentCapabilityInput = new ConnectApi.ContentCapabilityInput();
commentCapabilitiesInput.content = contentCapabilityInput;
contentCapabilityInput.title = 'Title';
commentInput.capabilities = commentCapabilitiesInput;
String text = 'These are the contents of the new file.';
Blob myBlob = Blob.valueOf(text);
ConnectApi.BinaryInput binInput = new ConnectApi.BinaryInput(myBlob, 'text/plain', 'fileName');
ConnectApi.Comment commentRep = ConnectApi.ChatterFeeds.postCommentToFeedElement(Network.getNetworkId(), feedElementId, commentInput, binInput);