Google App Engine when trying to upload static .mp3 files through app.yaml

I'm trying to make some audio available to my web app that I can access through JavaScript.

The audio file (~20 kB in size) is located in the root/sounds folder; like I do with other static files (css stylesheets, JavaScript files, images, etc.), I added this to my app.yaml:

- url: /sounds
  static_dir: sounds

But then when I try to deploy my app, the tool throws this error:

2015-08-02 17:34:35,805 ERROR appcfg.py:2621 An unexpected error occurred. Aborting. 
Traceback (most recent call last):
  File "C:\Program Files (x86)\Google\google_appengine\google\appengine\tools\appcfg.py", line 2459, in DoUpload
    self._UploadMissingFiles(missing_files, openfunc)
  File "C:\Program Files (x86)\Google\google_appengine\google\appengine\tools\appcfg.py", line 2600, in _UploadMissingFiles
    self.blob_batcher.Flush()
  File "C:\Program Files (x86)\Google\google_appengine\google\appengine\tools\appcfg.py", line 1464, in Flush
    self.SendBatch()
  File "C:\Program Files (x86)\Google\google_appengine\google\appengine\tools\appcfg.py", line 1423, in SendBatch
    payload,
UnicodeDecodeError: 'ascii' codec can't decode byte 0xff in position 40: ordinal not in range(128)

Is this a Google App Engine bug, or am I doing something wrong?

I'm on Windows 10, if that makes a difference.