S3 filename generated by ``generateKey`` option is ignored by Parse Server

[mman]

If you use the generateKey to customize a filename before uploading a file to S3, the newly generated filename is then ignored and not properly propagated back to the Parse Server to be stored in the fields referencing that file.

The only way to make the generateKey option work is to turn on ParseServer preserveFilenames option that will leave all the logic of generating proper file names on the client.

See parse-community/parse-server#6518

The proposal would be to modify validateFilename in file adapters to not return error but rather a modified filename. That can in turn reuse the generateKey option.

[mman]

https://github.com/parse-community/parse-server-s3-adapter/blob/master/index.js#L113 will have response.Key contain the real filename uploaded to S3. There is currently no way to propagate that officially to the Parse Sever.

[mman]

Once Parse Server 4.2 with support for file triggers is out, I propose to retire the generateKey option to S3 adapter entirely since it is broken anyway. Thoughts?

parse-community/parse-server#6344