Upload of application/zip file with non-".zip" extension leads to JS error

[TwizzyDizzy]

Rocket.Chat Version: 0.57.1 + 0.56.0 (tested on both, the former is the dev environment, the latter is the corresponding live environment)
Running Instances: 1
DB Replicaset OpLog: disabled
Node Version: 4.8.3

Upload of filetype .sketch fails. This seems to be a normal zip file with the extension ".sketch".

The mime-type application/zip is enabled for uploads. If the attached .sketch file (example file: Untitled.zip - rename to "notrelevant.sketch" first, before trying to upload) is uploaded (drag&drop), there is not UI feedback but the attached error in the JavaScript console.

Expected behaviour would be to tell the user "this is no valid filetype" or to show the "name this upload"-dialogue. Neither happens.

Cheers
Thomas