model: Alter column to LONGBLOB

[YoshihitoAso]

close #1416

BEFORE

+--------------+--------------+------+-----+---------+----------------+
| Field        | Type         | Null | Key | Default | Extra          |
+--------------+--------------+------+-----+---------+----------------+
| created      | datetime     | YES  |     | NULL    |                |
| modified     | datetime     | YES  |     | NULL    |                |
| id           | bigint       | NO   | PRI | NULL    | auto_increment |
| to_email     | varchar(256) | NO   |     | NULL    |                |
| subject      | varchar(100) | NO   |     | NULL    |                |
| text_content | text         | NO   |     | NULL    |                |
| html_content | text         | NO   |     | NULL    |                |
| file_name    | varchar(255) | YES  |     | NULL    |                |
| file_content | blob         | YES  |     | NULL    |                |
+--------------+--------------+------+-----+---------+----------------+

AFTER

+--------------+--------------+------+-----+---------+----------------+
| Field        | Type         | Null | Key | Default | Extra          |
+--------------+--------------+------+-----+---------+----------------+
| created      | datetime     | YES  |     | NULL    |                |
| modified     | datetime     | YES  |     | NULL    |                |
| id           | bigint       | NO   | PRI | NULL    | auto_increment |
| to_email     | varchar(256) | NO   |     | NULL    |                |
| subject      | varchar(100) | NO   |     | NULL    |                |
| text_content | text         | NO   |     | NULL    |                |
| html_content | text         | NO   |     | NULL    |                |
| file_name    | varchar(255) | YES  |     | NULL    |                |
| file_content | longblob     | YES  |     | NULL    |                |
+--------------+--------------+------+-----+---------+----------------+