What is the relationship between metadata and a file extension?

Metadata is a term that refers to data that describe other data; while metadata is a sort of data in itself, in conjunction to other data it is called metadata.

There are several types of metadata, depending on what type of information they yield. They could describe the nature of the data, they could outline their structure or they could notify us of administrative issues.

The file extension is a type of structural metadata; It simply conveys immediately not the data, but the way the data is saved in an electronic form. Hence, file extensions are a part, a subset of the metadata that can be gathered for files.