fix: align uploadAttachment with backend API contract

- Change FormData field name from "file" to "data" (backend expects "data")
- Pass conversation_id as query parameter instead of form field
- Add content_type and created_at to AttachmentData interface to match
  backend response shape

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

frontend/lib/api.ts

@@ -79,9 +79,11 @@ export async function fetchTicketSummary(): Promise<TicketSummaryData> {
 
 export interface AttachmentData {
   id: string;
-  blob_url: string;
   filename: string;
+  content_type: string;
+  blob_url: string;
   size_bytes: number;
+  created_at: string;
 }
 
 export async function uploadAttachment(
@@ -89,12 +91,10 @@ export async function uploadAttachment(
   conversationId?: string,
 ): Promise<AttachmentData> {
   const form = new FormData();
-  form.append("file", file);
-  if (conversationId) {
-    form.append("conversation_id", conversationId);
-  }
+  form.append("data", file);
 
-  const res = await fetch(`${API_URL}/api/attachments/upload`, {
+  const params = conversationId ? `?conversation_id=${conversationId}` : "";
+  const res = await fetch(`${API_URL}/api/attachments/upload${params}`, {
     method: "POST",
     body: form,
   });