feat(base64): optionally use base64 encoding for file content

[rohanku]

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Fixes #506 🦕

[google-cla]

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[chingor13]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

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CL needs to be signed before we can review