parse_multipart crashes with KeyError when Content-Type header is missing in multipart body part

[23Skidoo]

Bug Description

parse_multipart in testbench/common.py (line 307) crashes with a KeyError when a multipart upload body part does not include a Content-Type header.

Steps to Reproduce

Send a multipart upload request where the media body part omits the Content-Type header. This is what the google-cloud-storage Rust crate does when calling write_object(...).send_buffered().

Error

File "/opt/storage-testbench/testbench/common.py", line 307, in parse_multipart
    content_type = {"content-type": headers[content_type_key].decode("utf-8")}
                                    ~~~~~~~^^^^^^^^^^^^^^^^^^
KeyError: b'content-type'

Expected Behavior

Per RFC 2046, a missing Content-Type in a body part should default to text/plain (or application/octet-stream for GCS media). The testbench should handle this gracefully rather than crashing.

The real GCS API accepts these uploads without issue.

Environment

• storage-testbench Docker image: gcr.io/cloud-devrel-public-resources/storage-testbench:latest (pulled 2026-05-15)
• Client: google-cloud-storage Rust crate (google-cloud-rust)

[23Skidoo]

Minimal curl reproducer

# Start the testbench
docker run --rm -d --name storage-testbench -p 9023:9000 \
  gcr.io/cloud-devrel-public-resources/storage-testbench:latest

# Wait for it to be ready
curl -sf --retry 5 --retry-connrefused --retry-delay 1 http://localhost:9023/

# Create a bucket
curl -s -X POST "http://localhost:9023/storage/v1/b" \
  -H "Content-Type: application/json" \
  -d '{"name":"repro-bucket"}'

# Multipart upload where the media part has NO Content-Type header
# (the metadata part has Content-Type: application/json, but the media part omits it)
printf '--boundary123\r\nContent-Type: application/json\r\n\r\n{"name":"test.txt"}\r\n--boundary123\r\n\r\nhello world\r\n--boundary123--\r\n' | \
  curl -s -X POST "http://localhost:9023/upload/storage/v1/b/repro-bucket/o?uploadType=multipart" \
    -H "Content-Type: multipart/related; boundary=boundary123" \
    --data-binary @-

Result: HTTP 500 with KeyError: b'content-type' traceback.

Expected: HTTP 200 (the real GCS API accepts multipart uploads where the media part omits Content-Type).