event: Explicitly ignore attachment events

[gnprice]

Manually tested that uploading an image before this commit generates
an error "Unhandled Zulip API event" on the debug console, and after
this commit no longer does so.

Fixes: #4687

[chrisbobbe]

Hmm, quoting that CI failure (in our native tests):

FAILURE: Build failed with an exception.

* What went wrong:
A problem occurred configuring project ':rn-fetch-blob'.
> Could not resolve all artifacts for configuration ':rn-fetch-blob:classpath'.
   > Could not resolve com.android.tools.build:gradle:3.2.1.
     Required by:
         project :rn-fetch-blob
      > Could not resolve com.android.tools.build:gradle:3.2.1.
         > Could not get resource 'https://jcenter.bintray.com/com/android/tools/build/gradle/3.2.1/gradle-3.2.1.pom'.
            > Could not HEAD 'https://jcenter.bintray.com/com/android/tools/build/gradle/3.2.1/gradle-3.2.1.pom'. Received status code 502 from server: Bad Gateway

Looks temporary. I've hit "rerun all jobs".

Otherwise LGTM!

[gnprice]

Hmm, odd. I see the same in the rerun, too.

To double-check things, I ran tools/test --all locally, and it passes. (I probably have that dependency in my local Gradle cache, so that I'm not affected by that jcenter.bintray.com outage.) So, merging. Thanks for the review!