Not possible to handly attachments

Hi,

the persistence fails when sending emails with attachemnts with the error message:

```
(1/3) #1406 Doctrine\DBAL\Exception\DriverException

An exception occurred while executing a query: Data too long for column 'message' at row 1
```
It probably makes sense to either catch that error and have the mail be sent anyways and/or extend the message table to mediumtext