Add ContentType to s3.send

[KevinPayravi]

When uploading thumbnails to S3, no ContentType value was provided. This causes the ContentType to default to application/octet-stream.

This can cause downstream issues, if that content type makes it all the way to an enduser (e.g. if a user views a thumbnail directly, the browser may prompt them to download the file instead of simply displaying it).

This PR adds ContentType: "image/jpeg" to the s3.send command.

[coderabbitai]

Walkthrough

The Lambda thumbnail generator now explicitly sets ContentType: "image/jpeg" when uploading resized thumbnails to S3. This ensures the stored objects carry the correct MIME type metadata in the dpla-thumbnails bucket. Download, resize, and error handling flows remain unchanged.

Changes

S3 Upload Configuration

Layer / File(s)	Summary
S3 Upload ContentType thumbnailer/app.mjs	PutObjectCommand now includes ContentType: "image/jpeg" parameter for thumbnail uploads.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~2 minutes

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title 'Add ContentType to s3.send' directly and specifically describes the main change: adding ContentType metadata to the S3 upload operation for thumbnails.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch add-content-type

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[DominicBM]

Looks good!