https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24919/diffs adds
support for Rails sending the right `Content-Disposition` header when
there are non-ASCII filenames (e.g. `テスト.txt`). That example file
would render the following Content-Disposition:

```
attachment; filename="テスト.txt", filename*=UTF-8''%E3%83%86%E3%82%B9%E3%83%88.txt
```

Since mime.ParseMediaType appears to strip this header to a canonical
filename, it's not possible to use this to generate the right
Content-Disposition. Instead, we can preserve the existing
Content-Disposition and convert types just by doing a search and replace
(i.e. change `attachment` to `inline` and vice versa when necessary).

Closes https://gitlab.com/gitlab-org/gitlab-workhorse/issues/207