Unable to set `charset` when mime-types are guessed (S3)

We are syncing a directory of various file types to an S3 bucket and `aws-cli` is correctly guessing the mime-types, however in our case it's important that it also append the `charset`. For example the guessed content-type for `index.html` might look like this:

```
Content-Type: text/html
```

But we'd like a way to be able to tell `aws-cli` that the `charset` for all the synced files is `UTF8` for instance:

```
Content-Type: text/html; charset=utf-8
```

Version: `aws-cli/1.7.26 Python/2.7.6 Darwin/14.4.0`