filename extension vs file name extension

filename extension and file name extension both are nouns.

Word	Noun	Adjective	Verb	Adverb
filename extension	Yes	No	No	No
file name extension	Yes	No	No	No

As nouns, filename extension and file name extension are synonyms defined as:

filename extension and file name extension: a string of characters beginning with a period and followed by one or more letters; the optional second part of a PC computer filename

Other synonyms of filename extension include extension.

filename extension (noun)	file name extension (noun)
a string of characters beginning with a period and followed by one or more letters; the optional second part of a PC computer filename	a string of characters beginning with a period and followed by one or more letters; the optional second part of a PC computer filename