If this is a video file, I suggest using Microsoft's Windows Media Player to open the file. Since you don't have an extension, WMP will give you a warning that the file content type doesn't match the file extension, and will then ask you if you want to open the file. If you say yes, WMP will attempt to open the file using a set of generic decoders. If WMP still can not open the file, you'll get another prompt asking if you want to check the internet for the proper en/decoders. This will in-turn do a sophisticated search for the proper file type based on the actual file content, not the file extension.
Finally, this is not a mIRC problem, and really shouldn't have been posted in this forum.