.ico files are not working.

[btimby]

preview-server emits an error when trying to preview a .ico file.

MissingDelegateError: no decode delegate for this image format `' @ error/constitute.c/ReadImage/504
  File "preview/__init__.py", line 265, in handler
    await generate(obj)
  File "concurrent/futures/thread.py", line 56, in run
    result = self.fn(*self.args, **self.kwargs)
  File "preview/preview.py", line 60, in generate
    Backend.preview(obj)
  File "preview/preview.py", line 46, in preview
    return _preview(be, obj)
  File "preview/preview.py", line 30, in _preview
    be.preview(obj)
  File "preview/backends/base.py", line 25, in preview
    return method(obj)
  File "preview/utils.py", line 55, in inner
    return f(*args, **kwargs)
  File "preview/backends/image.py", line 79, in _preview_image
    path = resize_image(obj.src.path, obj.width, obj.height)
  File "preview/backends/image.py", line 22, in resize_image
    with Image(filename=path, resolution=300) as s:
  File "wand/image.py", line 8368, in __init__
    units=units)
  File "wand/image.py", line 8841, in read
    self.raise_exception()
  File "wand/resource.py", line 230, in raise_exception
    raise e

https://sentry.lumanox.io/organizations/lumanox/issues/2885/?project=14&query=is%3Aunresolved