AttributeError: 'NoneType' object has no attribute 'read'

https://sentry.softwareheritage.org/organizations/swh/issues/104787/?referrer=phabricator_plugin

AttributeError: 'NoneType' object has no attribute 'read'
(7 additional frame(s) were not displayed)
...
  File "rest_framework/views.py", line 492, in dispatch
    response = handler(request, *args, **kwargs)
  File "swh/deposit/api/common.py", line 1153, in post
    request, headers, collection_name, deposit
  File "swh/deposit/api/collection.py", line 138, in process_post
    receipt = self._multipart_upload(req, headers, collection_name, deposit)
  File "swh/deposit/api/common.py", line 598, in _multipart_upload
    data["application/atom+xml"]
  File "swh/deposit/api/common.py", line 508, in _read_metadata
    raw_metadata = metadata_stream.read()

happens because application/xml+atom was provided instead of application/atom+xml.

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Migrated from T4622 (view on Phabricator)