In addition to the name of the file there is always a file name extension that appears at the end of the file like this:

some_filename.file_name_extension

The file_name_extension always appears following the filename and the two parts are separated by a single period.

The file name extension identifies the type of the file. There are many types of files and it is important to know that different types of files are used for very specific purposes.

The primary distinction among file types is those that are "binary" which can only be interpreted by a specifc compyter program. All other file types are considered text as they can be read and interpreted by ANY text editor.and those that are

Some common file types (from http://it.nmu.edu/docs/common-windows-file-extensions)