A file called mybackup is found while investigating a USB pen drive.

The file command is used to determine the type of data contained in a computer file. It performs a series of tests on the file and prints a description of the file type based on the first successful test. The file command can take one or more file names as its arguments. For example, file mybackup will display the file type of mybackup. The other options are not valid commands for finding the file type. References:

file command in Linux with examples

file (command) - Wikipedia

Linux File Command