feat: support mediaType in watched attachments

[VIVAAN-DHAWAN]

Closes #922

Summary

• Add optional mediaType to WatchedAttachmentItem for filename and fileExtension variants.
• Preserve watched mediaType when creating QUEUED_DOWNLOAD attachment records.
• Document the new field and add a changeset for @powersync/common.

Verification

• Reproduced the bug with the new regression: mediaType was null before the fix.
• pnpm --filter @powersync/common build
• pnpm --filter @powersync/node build
• pnpm exec vitest run packages/node/tests/attachments.test.ts
• pnpm exec prettier --check .changeset/fuzzy-cameras-sip.md packages/common/src/attachments/AttachmentQueue.ts packages/common/src/attachments/WatchedAttachmentItem.ts packages/node/tests/attachments.test.ts
• git diff --check

[changeset-bot]

🦋 Changeset detected

Latest commit: eacc0ce

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 7 packages

Name	Type
@powersync/common	Patch
@powersync/node	Patch
@powersync/op-sqlite	Patch
@powersync/react-native	Patch
@powersync/tanstack-react-query	Patch
@powersync/web	Patch
@powersync/diagnostics-app	Patch

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