refactor: extract testable image attachment capture from the composer

The paste/drop/file capture logic (supported-type filtering, unnamed-file
extension fallback, per-file error collection) lived inside PromptEditor,
mixing browser side effects with branching that had no tests.

Move it into a pure promptAttachmentCapture module with the byte reader
injected, so the FileReader side effect stays at the component boundary
and the orchestration is unit-tested. Also removes a duplicated mime->ext
fallback in favour of the shared extensionForImageMimeType helper.

src/client/src/components/PromptEditor.ts

@@ -7,7 +7,7 @@ import { LitElement, html, type PropertyValues } from "lit";
 import { customElement, property, query, state } from "lit/decorators.js";
 import { api, type FileSuggestion, type PromptAttachment, type SessionStatus, type SlashCommand } from "../api";
 import type { PromptAttachmentDelivery } from "../../../shared/apiTypes";
-import { isSupportedImageMimeType } from "../../../shared/promptAttachments";
+import { captureImageAttachments } from "../promptAttachmentCapture";
 import { inputModeForDraft } from "../inputModes";
 import { machineSessionKey } from "../machineKeys";
 import { detectPromptCompletionTrigger, fileCompletionInsertText, type PromptCompletionTrigger } from "../promptCompletions";
@@ -187,24 +187,11 @@ export class PromptEditor extends LitElement {
 
   private async addAttachmentFiles(files: File[]) {
     this.attachmentError = undefined;
-    for (const file of files) {
-      if (!isSupportedImageMimeType(file.type)) {
-        this.attachmentError = "Only PNG, JPEG, GIF, and WebP images are supported.";
-        continue;
-      }
-      try {
-        const data = await readFileAsBase64(file);
-        this.attachments = [...this.attachments, {
-          id: `attachment-${String(++this.attachmentSeq)}`,
-          name: file.name !== "" ? file.name : `pasted-image.${file.type.split("/")[1] ?? "png"}`,
-          mimeType: file.type,
-          data,
-          size: file.size,
-        }];
-      } catch {
-        this.attachmentError = "Failed to read an attachment.";
-      }
+    const { attachments, error } = await captureImageAttachments(files, readFileAsBase64);
+    if (attachments.length > 0) {
+      this.attachments = [...this.attachments, ...attachments.map((attachment) => ({ id: `attachment-${String(++this.attachmentSeq)}`, ...attachment }))];
     }
+    if (error !== undefined) this.attachmentError = error;
   }
 
   private currentAttachments(): PromptAttachment[] {

src/client/src/promptAttachmentCapture.test.ts

@@ -0,0 +1,51 @@
+import { describe, expect, it } from "vitest";
+import { captureImageAttachments, READ_FAILURE_MESSAGE, UNSUPPORTED_IMAGE_MESSAGE, type CapturableFile } from "./promptAttachmentCapture";
+
+function file(name: string, type: string, size = 10): CapturableFile {
+  return { name, type, size };
+}
+
+describe("captureImageAttachments", () => {
+  it("reads supported images as base64 attachments", async () => {
+    const result = await captureImageAttachments(
+      [file("shot.png", "image/png"), file("pic.webp", "image/webp")],
+      (f) => Promise.resolve(`data-for-${f.name}`),
+    );
+
+    expect(result.error).toBeUndefined();
+    expect(result.attachments).toEqual([
+      { name: "shot.png", mimeType: "image/png", data: "data-for-shot.png", size: 10 },
+      { name: "pic.webp", mimeType: "image/webp", data: "data-for-pic.webp", size: 10 },
+    ]);
+  });
+
+  it("derives a name from the mime type when the file is unnamed", async () => {
+    const result = await captureImageAttachments([file("", "image/jpeg")], () => Promise.resolve("x"));
+    expect(result.attachments[0]?.name).toBe("pasted-image.jpg");
+  });
+
+  it("skips unsupported types and reports a single error while keeping valid ones", async () => {
+    const result = await captureImageAttachments(
+      [file("doc.pdf", "application/pdf"), file("ok.gif", "image/gif")],
+      () => Promise.resolve("x"),
+    );
+
+    expect(result.error).toBe(UNSUPPORTED_IMAGE_MESSAGE);
+    expect(result.attachments.map((attachment) => attachment.name)).toEqual(["ok.gif"]);
+  });
+
+  it("reports a read failure without dropping other attachments", async () => {
+    const result = await captureImageAttachments(
+      [file("bad.png", "image/png"), file("good.png", "image/png")],
+      (f) => f.name === "bad.png" ? Promise.reject(new Error("boom")) : Promise.resolve("ok"),
+    );
+
+    expect(result.error).toBe(READ_FAILURE_MESSAGE);
+    expect(result.attachments.map((attachment) => attachment.name)).toEqual(["good.png"]);
+  });
+
+  it("returns no attachments and no error for an empty batch", async () => {
+    const result = await captureImageAttachments([], () => Promise.resolve("x"));
+    expect(result).toEqual({ attachments: [] });
+  });
+});

src/client/src/promptAttachmentCapture.ts

@@ -0,0 +1,61 @@
+import { extensionForImageMimeType, isSupportedImageMimeType } from "../../shared/promptAttachments";
+
+/**
+ * Minimal view of a browser File needed to capture an attachment. Keeping this
+ * structural (rather than depending on the DOM `File` type) lets the capture
+ * logic be unit-tested without a browser environment.
+ */
+export interface CapturableFile {
+  name: string;
+  type: string;
+  size: number;
+}
+
+export interface CapturedAttachment {
+  name: string;
+  mimeType: string;
+  /** Base64 payload without the data: URL prefix. */
+  data: string;
+  size: number;
+}
+
+export interface CaptureResult {
+  attachments: CapturedAttachment[];
+  error?: string;
+}
+
+export const UNSUPPORTED_IMAGE_MESSAGE = "Only PNG, JPEG, GIF, and WebP images are supported.";
+export const READ_FAILURE_MESSAGE = "Failed to read an attachment.";
+
+/**
+ * Validate a batch of files and read the supported images as base64.
+ *
+ * Pure orchestration: the actual byte reading is injected so the side effect
+ * (FileReader/Blob access) stays at the component boundary and tests can supply
+ * a fake reader. Unsupported types and read failures are collected into a single
+ * user-facing error while still returning every attachment that did succeed.
+ */
+export async function captureImageAttachments<T extends CapturableFile>(
+  files: readonly T[],
+  readBase64: (file: T) => Promise<string>,
+): Promise<CaptureResult> {
+  const attachments: CapturedAttachment[] = [];
+  let error: string | undefined;
+  for (const file of files) {
+    if (!isSupportedImageMimeType(file.type)) {
+      error = UNSUPPORTED_IMAGE_MESSAGE;
+      continue;
+    }
+    try {
+      const data = await readBase64(file);
+      attachments.push({ name: attachmentName(file), mimeType: file.type, data, size: file.size });
+    } catch {
+      error = READ_FAILURE_MESSAGE;
+    }
+  }
+  return { attachments, ...(error === undefined ? {} : { error }) };
+}
+
+function attachmentName(file: CapturableFile): string {
+  return file.name !== "" ? file.name : `pasted-image.${extensionForImageMimeType(file.type)}`;
+}