code: async function upload_file(file, { upload_folder, log })
  {
  	if (log)
  	{
  		log.debug(`Uploading: ${file.filename}`)
  	}
  
  	// Generate random unique filename
  	const file_name = await generate_unique_filename(upload_folder,
  	{
  		on_collision: (file_name) =>
  		{
  			log.info(`Generate unique file name: collision for "${file_name}". Taking another try.`)
  		}
  	})
  
  	// dot_extension: path.extname(file.filename)
  
  	const output_file = path.join(upload_folder, file_name)
  
  	// Write the file to disk
  	return await new Promise((resolve, reject) =>
  	{
  		// Ensure the `upload_folder` exists
  		// (just an extra precaution)
  		fs.ensureDir(upload_folder, (error) =>
  		{
  			if (error)
  			{
  				return reject(error)
  			}
  
  			// Open output file stream
  			const stream = fs.createWriteStream(output_file)
  
  			// Pipe file contents to disk
  			file.pipe(stream)
  				.on('finish', () => resolve(path.relative(upload_folder, output_file)))
  				.on('error', error => reject(error))
  		})
  	})
  }
docs: Handles file upload. Takes a `file` busboy file object along with `upload_folder` option. Writes the `file` stream to `upload_folder` naming it with a randomly generated filename.  Returns a Promise resolving to the randomly generated filename.