Don't try to set Content-Type if no mimetype is available

[lalten]

This phase:

def attach(test_api: htf.TestApi) -> None:
    test_api.attach("file.unknown", b"I'm data")

will work, but when you try to download an attachment that has an extension that https://github.com/google/openhtf/blob/c669a9cc64fd2c4e86cd3cc6431c08545156d483/openhtf/core/test_state.py#L669-L670 does not recognize, the mimetype is set to None (null in json). This makes Tornado fail on attachment download when it sets the Content-Type of the response:

Traceback (most recent call last):
  File "path/to/tornado/web.py", line 1702, in _execute
    result = method(*self.path_args, **self.path_kwargs)
  File "path/to/openhtf/output/servers/station_server.py", line 369, in get
    self.set_header('Content-Type', attachment.mimetype)
  File "path/to/tornado/web.py", line 374, in set_header
    self._headers[name] = self._convert_header_value(value)
  File "path/to/tornado/web.py", line 416, in _convert_header_value
    raise TypeError("Unsupported header value %r" % value)
TypeError: Unsupported header value None

The simple fix is to only send the Content-Type header when we actually know it.

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This change is 

[dbhatman]

Closing and reopening to trigger some automation.