Updated docs about custom onFile handler.

[twentyrogersc]

This addresses #29, where the documentation for onFile is incorrect - filePromises are not passed as an arg to a custom onFile handler.

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Happy to add documentation for how to add a custom filePromises, however I felt that this may overcomplicate/confuse initial readers - thoughts?

async function(ctx, next) {
  const files = []
  const { fields } = await asyncBusboy(ctx.req, {
    onFile: function(fieldname, file, filename, encoding, mimetype) {
      files.push(uploadFilesToS3(file));
    }
  });

  const uploads = await Promise.all(files)
  if ( !validate(fields, uploads) ) {
    return 'error';
  }
}

[codecov-io]

Codecov Report

Merging #30 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #30   +/-   ##
=======================================
  Coverage   93.81%   93.81%           
=======================================
  Files           1        1           
  Lines          97       97           
  Branches       11       11           
=======================================
  Hits           91       91           
  Misses          6        6

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[m4nuC]

Thank you @twentyrogersc !