Firebase Storage Product Manager here:
TL;DR: Use a UUID generator (in Android (UUID) and iOS (NSUUID) they are built in, in JS you can use something like this: Create GUID / UUID in JavaScript?), then append the file extension if you want to preserve it (split the file.name on '.' and get the last segment)
We didn't know which version of unique files developers would want (see below), since there are many, many use cases for this, so we decided to leave the choice up to developers.
images/uuid/image.png // option 1: clean name, under a UUID "folder"
image/uuid.png // option 2: unique name, same extension
images/uuid // option 3: no extension
It seems to me like this would be a reasonable thing to explain in our documentation though, so I'll file a bug internally to document it :)
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