Add 'application' file extension in example

[homersimpsons]

This is the longest extension generated by Faker (with 11 characters).

Even if it is not highlighted as the longest one it can give insights that extensions are not always 3/4 characters.

Number of characters for longest extension can be found from

$mimeTypes = Faker\Provider\File::$mimeTypes;

var_dump(max(array_map('strlen', array_merge(...array_values(array_map(fn ($e) => (array) $e, $mimeTypes))))));