@Wei WuListS3 Processor doesn't allow any upstream connections if you want to fetch some files from the S3 object without listS3 processor then you need to keep your filenames as attributes to the flowfile then use FetchS3 object processor to fetch the files from s3 bucket.
To get all the filenames from the S3 bucket you need to use some kind of RestApi call or shell script to list out all filenames from the Bucket then extract the filenames as attributes to the flowfile.
Then feed the connection to the FetchS3 processor to fetch actual file contents without ListS3 processor.
Refer to this link to get more context over these kind of usecase.