Cache headers not being set for webP images

[norahine]

I've added a caching policy for webP images (see code below), but the headers are not being set on webP images in S3. Note that they are being set on all the other types of files just fine.

Copied from config.rb:

cache_max_age = 60 * 60 * 24 * 365

caching_policy 'text/css', max_age: cache_max_age
caching_policy 'application/javascript', max_age: cache_max_age
caching_policy 'image/png', max_age: cache_max_age
caching_policy 'image/jpeg', max_age: cache_max_age
caching_policy 'image/gif', max_age: cache_max_age
caching_policy 'image/webp', max_age: cache_max_age
caching_policy 'image/svg+xml', max_age: cache_max_age
caching_policy 'text/html', max_age: 0

middleman version: (3.4.1)
middleman-s3_sync version: (3.3.10)

Does anybody have an idea what might be going wrong?