feat: normalize and filter chat attachments

Attachment pickers now share an allowlist for supported file types
Local attachments are normalized to consistent MIME types before upload
VS Code file picker now respects extension filters and larger files are allowed

packages/ui/src/lib/api/types.ts

@@ -753,7 +753,7 @@ export interface VSCodeAPI {
   executeCommand(command: string, ...args: unknown[]): Promise<unknown>;
   openAgentManager(): Promise<void>;
   openExternalUrl(url: string): Promise<void>;
-  pickFiles?(): Promise<unknown>;
+  pickFiles?(options?: { extensions?: string[] }): Promise<unknown>;
   saveImage?(payload: unknown): Promise<unknown>;
   saveMarkdown?(payload: unknown): Promise<unknown>;
 }