拡張子

かくちょうし

Common word

noun

•

filename extension(computer term)

拡	broaden, extend, expand, enlarge
張	lengthen, counter for bows & stringed instruments, stretch, spread, put up (tent)
子	child, sign of the rat, 11PM-1AM, first sign of Chinese zodiac

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かくちょうし

つ

よ

With Windows, you have to have extensions or it won't read your files