Find mail part recursively

I'm getting a `NoMethodError in Rails::Mailers#preview` when I try to preview an email view that has a attachment. 

More precisely:
```ruby 
undefined method `mime_type' for #<Array:0x007f6469451cd8>
```

 ActionMailer  generate a `multipart/alternative` email when use `attachments`.  It is necessary to look for mail part recursively.
this line shouldn't be needed unless `find_part` can return `false`