Create files

This creates the Attachment model that contains the file upload.
You never defined `@attachment` in `DatasetsController#show`. If you want to use `attachment`, which is the block argument that you defined on line 16, then you'll need to indent this line, because Haml is indent-based.

Normal Ruby:

``` ruby
@things.each do |thing|
  puts thing.to_s
end
```

Haml:

``` haml
= @things.each do |thing|
  = thing
-# no `end`!
```

Note that in normal Ruby, you don't _need_ to indent the line where you call `thing`, because the whitespace is (mostly) insignificant in Ruby. But in Haml, you need to be tidy, because whitespace is very significant.