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.