Today, I would like to highlight the usefulness of the file utility. It is a command-line utility that is common on Unix systems. File will take one or more files and try to determine what type of file they are by examining their contents. It's the most accurate general-purpose tool of its type that I have used. It can be very helpful in identifying file types when there is no extension in the file name. Since it works off of the file contents rather than just the extension, file can tell you, for instance, that the a file called "picture.jpg" is actually a PNG with an incorrect file extension.

Invocation:

file file1 [file2 file3 ...]