What does FTS_ file mean after converting from MyISAM to InnoDB?
These files are related to the new FULLTEXT index support in InnoDB (starting with MySQL 5.6). Removing them is not a good idea for sure.

Unlike with MyISAM, where FULLTEXT indexes are stored in the tables .MYI file together with all other indexes fulltext indexes in InnoDB are implemented using several internal helper tables which show up as separate .idb table space files in innodb_file_per_table mode:

The "...INDEX_1.ibd" to "...INDEX_6.ibd" table files contain the actual fulltext index, split up into 6 partitions. The "...CONFIG.idb" stores meta information about the fulltext index and the "...DELETED..." files are needed for housekeeping of deleted data. 

For more details see

http://dev.mysql.com/doc/refman/5.6/en/innodb-fulltext-index.html#idm140684547617952