Drop tablespace and flashback : ORA-01245: offline file 7 will be lost if RESETLOGS is done

Hello,  I need your help to resolve a problem occur after flashback and open resetlogs;

Scenario :

   1 create tablespace TS1
   2  create restore point RP1;
   3 drop tablespace TS1 (datafile  '/c:/ts1.dbf')  including files
   4 restore database to RP1
   5 alter database open resetlogs failed with this error :
       ERROR at line 1:
         ORA-01245: offline file 7 will be lost if RESETLOGS is done
         ORA-01110: data file 7: '/c:/ts1.dbf'

  Can you help me please to fix this issue and to understand why it's occured.
 
  it's Ok for me if i lose the file 7
  Best Regards
 
  Dj