This exception is thrown in case of an invalid file name. A file name is invalid, if it contains a NUL character. Attackers might use this to circumvent security checks: For example, a malicious user might upload a file with the name "foo.exe\0.png". This file name might pass security checks (i.e. checks for the extension ".png"), while, depending on the underlying C library, it might create a file named "foo.exe", as the NUL character is the string terminator in C.