Here is the video version, if you prefer it:
The file command is used to guess the filetype based on files contents. (Ward, 2014)
Here is an example of its usage:
mislav@mislavovo-racunalo:~/Documents$ file ls.txt
ls.txt: UTF-8 Unicode text
Hope you learned something new!
References
Ward, B. (2014). How Linux Works: What Every Superuser Should Know (2nd ed.). No Starch Press. Page 20