Be more explicit about valid thumbnails

src/main.rs

@@ -246,7 +246,7 @@ async fn thumbnail(
 ) -> Result<HttpResponse, Error> {
     let (size, file) = parts.into_inner();
 
-    if (size % 100) == 0 {
+    if valid_thumbnail_size(size) {
         let url = CONFIG.upstream_thumbnail_url(size, &file);
 
         return image(url, req, client).await;
@@ -255,6 +255,10 @@ async fn thumbnail(
     Ok(to_404())
 }
 
+fn valid_thumbnail_size(size: u64) -> bool {
+    [40, 50, 80, 100, 200, 400, 800, 1200].contains(&size)
+}
+
 async fn full_res(
     filename: web::Path<String>,
     req: HttpRequest,