Some media never returning 304 Not Modified

Some media (seems to be icons) are not caching correctly. This is causing clients to unnecessarily re-download data. For example the Skype snap:

Icon doesn't cache correctly:

$ curl -v https://dashboard.snapcraft.io/site_media/appmedia/2018/12/Skype.png
...
HTTP/1.1 200 OK
ETag: 0eb79490aa87b6db57964275e8182da1
Cache-Control: public,max-age=31536000
...

$ curl -v -H 'If-None-Match: 0eb79490aa87b6db57964275e8182da1' https://dashboard.snapcraft.io/site_media/appmedia/2018/12/Skype.png
...
HTTP/1.1 200 OK
...

But a screenshot does:

$ curl -v http://dashboard.snapcraft.io/site_media/appmedia/2018/01/Linux_Screenshare-01.png
...
HTTP/1.1 200 OK
ETag: 15c7408f02db3af6a2469300b53f2473
Cache-Control: public,max-age=31536000
...

$ curl -v -H 'If-None-Match: 15c7408f02db3af6a2469300b53f2473' http://dashboard.snapcraft.io/site_media/appmedia/2018/01/Linux_Screenshare-01.png
...
HTTP/1.1 304 Not Modified
...