There is only one file type that uses the .NULL extension:
Null Ransomware Encrypted File
A file with a .null extension is a file that has been encrypted by the Null virus, which is a variant of Stop and Djvu ransomware that became prevalent in August 2017. It is encrypted with AES-256 algorithm so it is not possible to open the file by simply changing the .null file extension.
When the ransomware runs on a user's computer, it encrypts files on the computer and adds the .null extension onto the names of the files. The targeted files are typically documents, images, videos, and backup files, such as .DOCX, .JPG, .MP4, and .DB files. For example, a spreadsheet.xlsx file becomes spreadsheet.xlsx.null.
The virus then generates a pop-up window with several tabs explaining the hostile takeover of the user's files. The tabs include information about what occurred to the computer, the encryption used on the files, the list of encrypted files, how the user can recover his files, and how the user can make the Bitcoin ransom payment.
NOTE: Currently, there are several options for removing the Null virus, such as Malwarebytes Premium software, but there is no program available to effectively restore infected files. If the user has a recent backup of his files, he can perform a system restore to remove the virus but any changes made to files after the backup was made will be lost.
Updated: May 30, 2019