.FB! File

File extension fb! seems to be mainly associated with FlashGet, an internet download manager from Trend Media Corporation Limited. 

The fb! file contains incomplete download in progress. When the download is finished FlashGet will rename the FB! file extension back to its original.

How to open:

You cannot open .fb! files in any other program, except FlashGet. At best it will continue to download the file but may sometimes break and you have to initiate the download again.

How to convert:

Incomplete downloads in progress usually cannot be converted to anything else. You will need to finish the download first.