This post links to two Windows programs that can help you determine the extension of CHK-files.
A CHK-file is a file that is created when you run a disk repair tool like chkdsk, scandisk or other Windows disk tools.
Often these CHK-files are placed in a FOUND.000 folder and contains several FILE0001.CHK FILE0002.CHK … files and are being deleted by users because they don’t know what to do with them.
I’ve successfully used UnCHK and FileCHK in Windows Vista to recover many file extension/suffix and open them in the right program and higly recommend others to try it if they have lost important files.
Always remember to take backup before using these tools.