fix: encoding uploaded stage file as utf8

[hantmac]

Run c.execute("INSERT INTO example_table (a) VALUES", [('ó')]) will get error like :

"code":1046,"message":"Invalid value \'\xef\xbf\xbd\' for column 0 (field_1 String NULL): Invalid Utf8, cause: incomplete utf-8 byte sequence from index 0"