Malware
ATOMSILO
According to PCrisk, AtomSilo is a type of malware that blocks access to files by encrypting them and renames every encrypted file by appending the ".ATOMSILO" to its filename.
ATOMSILO is a Windows malware family.
Background
PCrisk describes AtomSilo as malware that denies access to files by encrypting them and tacks the ".ATOMSILO" extension onto each encrypted file's name. For example, "1.jpg" becomes "1.jpg.ATOMSILO", "2.jpg" becomes "2.jpg.ATOMSILO", and so forth. For its ransom note, AtomSilo drops a file named "README-FILE-#COMPUTER-NAME#-#CREATION-TIME#.hta".
Source: Malpedia (Fraunhofer FKIE).