docs: fix non-compiling attachment examples in the .NET README

[examon]

Fixes #2196

Problem

The three attachment examples in dotnet/README.md reference a type family that is no longer in the SDK - UserMessageDataAttachmentsItem, UserMessageDataAttachmentsItemFile, UserMessageDataAttachmentsItemBlob, and the enum UserMessageDataAttachmentsItemType. Copying any of them yields CS0246/CS0103.

This file is packed as the package README, so it is also the GitHub.Copilot.SDK NuGet page. The published 1.0.8 package ships these broken snippets today.

Attachments moved to a polymorphic model and the types were later renamed to Attachment*. The feature guide under docs/ was updated; this README was not, and documentation validation never compiles it because extraction is scoped to the docs/ directory (scripts/docs-validation/extract.ts resolves DOCS_DIR to docs and globs **/*.md relative to it).

Fix

Update the three examples to the shipped public API, aligning them with the C# blocks in docs/features/image-input.md that documentation validation already compiles:

• new List<UserMessageDataAttachmentsItem> becomes new List<Attachment>
• new UserMessageDataAttachmentsItemFile becomes new AttachmentFile
• new UserMessageDataAttachmentsItemBlob becomes new AttachmentBlob
• in ### File Attachments, the base type plus Type = UserMessageDataAttachmentsItemType.File becomes new AttachmentFile, and the discriminator assignment is dropped

Property sets are unchanged: AttachmentFile requires Path and DisplayName, both already present, and AttachmentBlob requires MimeType, also already present.

The discriminator line is removed rather than translated because the concrete subclass already carries it. AttachmentFile.Type is a get-only override that always returns "file" and is [JsonIgnore]d; the wire discriminator is emitted from [JsonPolymorphic]/[JsonDerivedType]. An explicit assignment would compile but be silently ignored, which is worse than not showing it.

Markdown only - no source, public API, schema, generated file, or codegen template is touched.

Verification

The examples were extracted from the README programmatically (not retyped) and compiled before and after:

Before, against published 1.0.8:

error CS0246: The type or namespace name 'UserMessageDataAttachmentsItem' could not be found
error CS0246: The type or namespace name 'UserMessageDataAttachmentsItemFile' could not be found
error CS0246: The type or namespace name 'UserMessageDataAttachmentsItemBlob' could not be found
error CS0103: The name 'UserMessageDataAttachmentsItemType' does not exist in the current context
Build FAILED.    7 Error(s)

After:

Build succeeded.
    0 Warning(s)
    0 Error(s)

Green against both a local build of this branch and the published 1.0.8 package, on netstandard2.0, net8.0 and net10.0. The complete ## Image Support fence also compiles as a single paste. Removing DisplayName from the corrected file example fails with CS9035, confirming the required-member check is live rather than vacuous.

Also verified: zero remaining occurrences of the old names in dotnet/README.md; the markdown token stream is unchanged apart from the one deleted line; dotnet format --verify-no-changes passes; and documentation validation still passes (55 C# files).

Round-tripping the corrected initializers through the real serializer gives {"type":"file",...} and {"type":"blob",...} and deserializes back to AttachmentFile/AttachmentBlob, confirming the dropped discriminator is supplied by the concrete type.

Scope

Deliberately limited to dotnet/README.md to keep this markdown-only.

dotnet/src/Session.cs has a related but distinct defect in the SendAsync XML <example>: new() inside a List<Attachment> initializer target-types to the base Attachment, which has no Path (CS0117), and it omits the required DisplayName. It uses current type names, so it is a different bug from the stale names fixed here. Happy to follow up on it, or on widening documentation validation to cover the per-binding READMEs, in separate PRs.

[Copilot]

Pull request overview

Updates the packaged .NET README to use the current attachment API, fixing examples reported in #2196.

Changes:

• Replaces removed attachment types with Attachment, AttachmentFile, and AttachmentBlob.
• Removes the obsolete explicit type discriminator.

Show a summary per file

File	Description
dotnet/README.md	Corrects three attachment examples to compile against the public SDK API.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 0
• Review effort level: Balanced

[github-actions]

Cross-SDK Consistency Review ✅

This PR makes a documentation-only fix to dotnet/README.md, updating three code examples to use the current public API type names (Attachment, AttachmentFile, AttachmentBlob) in place of the obsolete UserMessageDataAttachmentsItem* family.

No cross-SDK consistency concerns:

• No public API surface was changed — only markdown examples were corrected
• The fix aligns the README with the already-correct docs/features/image-input.md examples
• No equivalent documentation gap exists in other SDK READMEs (attachment type renaming was .NET-specific)

No action required in other SDK implementations.

Generated by SDK Consistency Review Agent for #2197 · sonnet46 15.9 AIC · ⌖ 5.37 AIC · ⊞ 6.5K · ◷

[SteveSandersonMS]

Thank you for identifying this. The reported documentation defect and the proposed corrections are valid. PR #2208 supersedes this PR by fixing the same README examples along with the adjacent invalid SendAsync IntelliSense/XML example, so consolidating on #2208 avoids duplicate overlapping changes. Thanks again for the contribution.