Cache preview frames for Discord video attachments

media-scrape/src/download.ts

@@ -193,18 +193,17 @@ const DISCORD_HOSTS = new Set(["cdn.discordapp.com", "media.discordapp.net"]);
 /**
  * The preview target for a media row.
  *
- * Attachments and stickers: their Discord-hosted URL, resized to a small WebP.
+ * Attachments and stickers: their Discord-hosted URL, resized to a small WebP. Discord's proxy
+ * also renders a frame for video files, so attachment videos get a poster for free.
  * Embeds: the stored thumbnail or image URL, used as-is (Discord-hosted ones are still resized).
- * Returns null when the row has no preview source.
+ * A bare embed URL is a page, never a preview. Returns null when the row has no preview source.
  */
 export function previewUrlFor(media: {
     url: string;
     kind: string;
     previewUrl?: string | null;
 }): string | null {
-    const candidate =
-        media.previewUrl ??
-        (media.kind === "image" || media.kind === "gif" ? media.url : null);
+    const candidate = media.previewUrl ?? media.url;
     if (!candidate) return null;
 
     try {
@@ -214,8 +213,13 @@ export function previewUrlFor(media: {
             url.searchParams.set("width", "320");
             url.searchParams.set("height", "320");
             url.searchParams.set("format", "webp");
+            return url.toString();
         }
-        return url.toString();
+
+        if (media.previewUrl || media.kind === "image" || media.kind === "gif") {
+            return url.toString();
+        }
+        return null;
     } catch {
         return null;
     }

media-scrape/src/store.ts

@@ -342,7 +342,8 @@ export class ScrapeStore {
      * embed thumbnails only when `includeEmbeds` is set (download mode).
      */
     pendingPreviews(limit = 5000, includeEmbeds = false): MediaRow[] {
-        const local = "(kind IN ('image', 'gif') AND (source IS NULL OR source != 'embed'))";
+        const local =
+            "((source IS NULL OR source != 'embed') AND kind IN ('image', 'gif', 'video'))";
         const condition = includeEmbeds
             ? `(${local} OR (source = 'embed' AND preview_url IS NOT NULL))`
             : local;

media-scrape/test/run.ts

@@ -522,6 +522,10 @@ void (async () => {
             "https://media.discordapp.net/attachments/1/2/pic.png?width=320&height=320&format=webp"
         );
         assert.equal(previewUrlFor({ url: "https://www.youtube.com/watch?v=x", kind: "video" }), null);
+        assert.equal(
+            previewUrlFor({ url: "https://cdn.discordapp.com/attachments/1/2/clip.mp4?ex=abc", kind: "video" }),
+            "https://media.discordapp.net/attachments/1/2/clip.mp4?ex=abc&width=320&height=320&format=webp"
+        );
         assert.equal(
             previewUrlFor({
                 url: "https://www.youtube.com/watch?v=x",
@@ -599,7 +603,10 @@ void (async () => {
             {
                 messages: [
                     message("600", {
-                        attachments: [attachment({ filename: "prev.png", contentType: "image/png" })],
+                        attachments: [
+                            attachment({ filename: "prev.png", contentType: "image/png" }),
+                            attachment({ filename: "clip.mp4", contentType: "video/mp4" })
+                        ],
                         embeds: [
                             {
                                 type: "video",
@@ -643,9 +650,10 @@ void (async () => {
 
         const meta = engine.get(started.id);
         assert.equal(meta?.state, "done");
-        assert.equal(meta?.previewed, 2);
+        assert.equal(meta?.previewed, 3);
         assert.equal(fetched.some(url => url.includes("ytimg.com")), true);
         assert.equal(fetched.some(url => url.includes("youtube.com/watch")), false);
+        assert.equal(fetched.some(url => url.includes("clip.mp4") && url.includes("format=webp")), true);
 
         const store = new ScrapeStore(join(root, started.id));
         const row = store.queryMedia({ kind: "image" })[0];