Digest checksum not properly working with file paths on push command

Hey guys,

I've noticed that if I pass a file path (absolute or relative) to push a given chart, the generated digest checksum is incorrect.

For example:

```sh
helm s3 push .deploy/my_chart.tgz my_repo
```

It will generate the wrong digest checksum, while:

```sh
helm s3 push my_chart.tgz my_repo
```

Creates the correct checksum.

Can someone please fix this?