Local file prints should be converted to base64

[Tres Finocchiaro]

Example:

var data = [{ type: 'pixel', format: 'pdf', flavor: 'file', data: '/Applications/QZ Tray.app/Contents/Resources/demo/pdf_sample.pdf' }];

Ideally, qz-tray.js should automatically detect file:/// and convert to Base64.

[Tres Finocchiaro]

Took a different approach via #1213