Hi thanks:
I re-downloaded those files and BTW I have updated qiime to 2019.1 but the same error there.
And my results are exactly the same as yours:
(/home/dy83/project/conda_envs/qiime2-2019.1) [dy83@ruddle2 q2-picrust2_test]$ qiime tools peek mammal_seqs.qza
UUID: 38a5d748-739e-44d1-b547-a35c0c8b8f29
Type: FeatureData[Sequence]
Data format: DNASequencesDirectoryFormat
(/home/dy83/project/conda_envs/qiime2-2019.1) [dy83@ruddle2 q2-picrust2_test]$ qiime tools peek reference.fna.qza
UUID: 87909524-fa4f-4b28-abd6-c7c89a3c7da7
Type: FeatureData[AlignedSequence]
Data format: AlignedDNASequencesDirectoryFormat
(/home/dy83/project/conda_envs/qiime2-2019.1) [dy83@ruddle2 q2-picrust2_test]$ qiime tools peek reference.tre.qza
UUID: f4e69728-a745-4069-b432-3f14951089fd
Type: Phylogeny[Rooted]
Data format: NewickDirectoryFormat