We forward emails with attachments for up to 10Mb per email (including message and attached files) and up to 50Mb for our premium members. Of course, the end mail server need to be able to accept this attachment size. For example, Gmail limits it to 25Mb.
We refuse some attachments for security reasons. The list is basically the same as the other main email providers.
Here are the extensions that are not allowed on ImprovMX: