email: Accept inline body with disposition params

A mail client may indicate a header parameter (such as Novell Groupwise, which may add something like

```
Content-Disposition: inline; modification-time: "Thu ..."
```

Previously, the system would not consider the content to be a candidate for the email body, because it has parameters to the content-disposition header.

This patch will still consider the part to be a candidate for the body if the mime types match and no filename is found in the content-disposition parameters.