Attaching base64 encoded data - should cucumber decide by itself for re-encoding ?

## Summary

This is a follow up from [cucumber-ruby#1443](https://github.com/cucumber/cucumber-ruby/issues/1443).

The user attaches a screenshot which is already encoded in `base64`. When we attach a file, we encode it once again as `base64` which makes the file "unreadable" (well, we can still decode it twice, but that's not what we want).

I want to add a bit more intelligence to `cucumber-ruby` so when we detect that the file is encoded in `base64`, we do not try to encode it once again.

How does this work for `cucumber-jvm` and `cucumber-js` ?
Shall I add an example in the CCK to ensure that all version of `cucumber` do the same thing ?

## Expected Behavior

Being smart :D

## Possible Solution

- test if the file is already encoded
- add a CCK example

## Steps to Reproduce (for bugs)

1. see [cucumber-ruby#1443](https://github.com/cucumber/cucumber-ruby/issues/1443)