file name extension vs filename extension

file name extension and filename extension both are nouns.

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

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

file name extension and filename 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 file name extension include extension.

file name extension (noun)	filename 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