Notes about the OK file extension:
To open this file type, a required software must be installed. Without it you only see a Windows message "The following file cannot be opened
" (Windows 7) or "How do you want to open this file?
" (Windows 10) if you double-click the file. There click on "More apps"
and choose a suitable software to view such files on your PC or from email attachments.
The OK file type is primarily associated with 'Dot OK' by John Oberschelp. Dot OK is a simple file naming system for appending a digest of a file's data to its name and verifying/removing it. To create an OK file create an MD5 hash of the concatination of the original file's name and the file's data. On receipt, if the hash matches the hash part of the .OK file name, then it is extremely likely that the data is unaltered. Files named this way would have the original file name, a period, an MD5 hash, and the .OK.