Monday API returns error on file uploading

when I tried to upload file via mondayapi ,it returns a response like this

{"errors":[{"message":"Argument 'column_id' on Field 'add_file_to_column' has an invalid value (Files). Expected type 'String!'.","locations":[{"line":1,"column":88}],"path":["mutation add_file","add_file_to_column","column_id"],"extensions":{"code":"argumentLiteralsIncompatible","typeName":"Field","argumentName":"column_id"}}],"account_id":11720007}

My request array is like this [query] => mutation add_file($file: File!) { complexity { query before after reset_in_x_seconds } add_file_to_column (file: $file, item_id:5982446753,column_id:File) { id } }

and data assigned to column_id is like below

if ($uvdocs) { $uvdcos_link = $uvdocs->user_identification_s3; $column_id = 'files2'; } else { return false; }

what is wrong here?