Add support for file inputs

[muan]

Fixes #2.

This adds support for <input type="file">, associating to <file-attachment> via an input attribute.

<file-attachment input="upload">
  <input type="file" id="upload" multiple>
</file-attachment>

I opted to name the attribute input instead of for to avoid confusion with <label for>. In the case of <label for>, input is implicitly associated through nesting, which isn't the true here.

[dgraham]

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