It really depends upon the file system and the operating system.
In POSIX and Unix, a file can be a directory, a plain file, a block device, a character device, a FIFO, a socket, a symbolic link, ... See POSIX stat & Linux stat(2), notice that file can have different types.
So if you want to speak of a file whose type is plain and which is a sequence of bytes, you say that is is a plain file.
If you want any file which is not a directory you could speak of a "non-directory" file (or maybe non-directory inode); it could be a plain file, a socket, a fifo, a symlink, etc...
Notice that POSIX and Linux don't know about folders (except on the desktop). They speak of directories.
Notice also that files are quite different on Windows, on MacOSX, on Linux, on POSIX....
Some experimental OSes don't even have files. They are persisting data in some other way. Historically, the very first editions of MS-DOS (or of MVS) did not had any directories (or folders).