You mentioned 'snapmanager' ?
Please take a look at the page 59 of this link:
https://library.netapp.com/ecm/ecm_download_file/ECMLP2680625 (Page 59)
According to SMSQL: SnapInfo directory holds the '.trb' files, these files are created by SMSQL, however if you run the transaction log backups using 'SQL Server Management Studio' outside of SMSQL application, then the files are named '.trn'.
Further it states: Transaction log backups created by SQL Server Management Studio have a default extension of .TRN. If you want to restore the transaction log backups from SnapManager, you must rename these .TRN files to .TRB and place them in the respective SnapInfo directory. After you place the transaction log backups in the correct SnapInfo directory, you can use SnapManager to restore from the logs.
Thanks!