fix(frontend): keep prompt attachments from breaking before upload (#1833)

* fix(frontend): preserve prompt attachment files during upload

* fix(frontend): harden prompt attachment fallback and tests

---------

Co-authored-by: Willem Jiang <willem.jiang@gmail.com>

frontend/src/core/threads/hooks.ts

@@ -14,7 +14,7 @@ import type { FileInMessage } from "../messages/utils";
 import type { LocalSettings } from "../settings";
 import { useUpdateSubtask } from "../tasks/context";
 import type { UploadedFileInfo } from "../uploads";
-import { uploadFiles } from "../uploads";
+import { promptInputFilePartToFile, uploadFiles } from "../uploads";
 
 import type { AgentThread, AgentThreadState } from "./types";
 
@@ -279,28 +279,9 @@ export function useThreadStream({
         if (message.files && message.files.length > 0) {
           setIsUploading(true);
           try {
-            // Convert FileUIPart to File objects by fetching blob URLs
-            const filePromises = message.files.map(async (fileUIPart) => {
-              if (fileUIPart.url && fileUIPart.filename) {
-                try {
-                  // Fetch the blob URL to get the file data
-                  const response = await fetch(fileUIPart.url);
-                  const blob = await response.blob();
-
-                  // Create a File object from the blob
-                  return new File([blob], fileUIPart.filename, {
-                    type: fileUIPart.mediaType || blob.type,
-                  });
-                } catch (error) {
-                  console.error(
-                    `Failed to fetch file ${fileUIPart.filename}:`,
-                    error,
-                  );
-                  return null;
-                }
-              }
-              return null;
-            });
+            const filePromises = message.files.map((fileUIPart) =>
+              promptInputFilePartToFile(fileUIPart),
+            );
 
             const conversionResults = await Promise.all(filePromises);
             const files = conversionResults.filter(
@@ -346,7 +327,6 @@ export function useThreadStream({
               });
             }
           } catch (error) {
-            console.error("Failed to upload files:", error);
             const errorMessage =
               error instanceof Error
                 ? error.message