Frustrating: in no way BlogEngine lets you use the <object> tag in a post without using any extension, it'll delete the whole <object> tag and everything within when you try to save the post.
Since I couldn't find a decent extension to add such a simple thing as Flash-files (*.swf) to a post, I decided to make one myself.

Download the extension here, unzip and upload FlashObjects.cs to the /App_Code/Extensions/ folder on your webserver.

Usage is quite simple. Lets say I want to show a flashobject named game.swf in a post, just do the following:
- upload game.swf to the folder where you store your flashfiles (default: /flash, you can change this location in the extension settings);
- put [ flashobject:game.swf ] in your post (without the spaces);
- enjoy! :-)