Data about a file is stored in several locations: system metadata (generated by file system or doc management of the OS), substantive metadata (information that defines modifications to a document), embedded metadata (information embedded by the application that creates or edits the file), external metadata (separate doc, for example, a database).
There are several ways to determine file type: extension, signatures and structure. The most reliable is the last one, since extentsions can be easily changed and signatures are not very reliable (polymorphic files). Some file types and signatures are listed here and in the book Digital Archaeology. Magic numbers can be viewed on a UNIX machine: /usr/share/file/magic.mgc (compiled list of magic numbers), /usr/share/file/magic (default list of magic numbers), /usr/share/file/magic.mime.mgc (default compiled list), /usr/share/file/magic.mime (default list). Use file command to determine file type.