fix: stop URL-encoding drag-out filenames and preserve Unicode letters

hooks/use-attachment-drag.ts

@@ -92,9 +92,11 @@ export function useAttachmentDrag(
         return;
       }
 
-      // `DownloadURL` format: <mime>:<filename>:<url>. Chromium reads this on
-      // drop and writes a real file at the destination.
-      e.dataTransfer.setData("DownloadURL", `${type}:${encodeURIComponent(name)}:${url}`);
+      // `DownloadURL` format: <mime>:<filename>:<url>. The filename must be
+      // raw - URL-encoding it lands literally on disk (`%20` instead of a
+      // space). Callers are expected to sanitise reserved chars (`:` etc.)
+      // beforehand.
+      e.dataTransfer.setData("DownloadURL", `${type}:${name}:${url}`);
       e.dataTransfer.effectAllowed = "copyMove";
     },
     [source.name, source.type, prefetch],

hooks/use-email-drag.ts

@@ -206,11 +206,14 @@ export function useEmailDrag({ email, sourceMailboxId, threadEmails }: UseEmailD
         const url = singleBlobUrlRef.current;
         if (url) {
           const name = emailExportFilename(emailsToDrag[0], emailTemplate);
-          // `DownloadURL` format: <mime>:<filename>:<url>. Chromium reads this
-          // on drop and writes a real file; Firefox/Safari ignore it.
+          // `DownloadURL` format: <mime>:<filename>:<url>. Chromium expects
+          // the filename raw - URL-encoding it ends up literally on disk
+          // (e.g. `%20` instead of a space). The sanitiser already removed
+          // `:` and other reserved chars, so embedding the name as-is is
+          // safe. Firefox/Safari ignore this entry entirely.
           e.dataTransfer.setData(
             "DownloadURL",
-            `message/rfc822:${encodeURIComponent(name)}:${url}`,
+            `message/rfc822:${name}:${url}`,
           );
         } else {
           // Not warmed up yet — kick off so the next attempt works. Don't
@@ -222,7 +225,7 @@ export function useEmailDrag({ email, sourceMailboxId, threadEmails }: UseEmailD
         if (ready) {
           e.dataTransfer.setData(
             "DownloadURL",
-            `application/zip:${encodeURIComponent(ready.name)}:${ready.url}`,
+            `application/zip:${ready.name}:${ready.url}`,
           );
         } else {
           // Kick off the bundle build for the next attempt.

lib/download-filename.ts

@@ -1,10 +1,11 @@
 import type { Email } from "@/lib/jmap/types";
 
-// Restrict filenames to ASCII letters/digits and a small set of safe
-// punctuation. Everything else collapses to `_`. Keeps names predictable
-// across Windows/macOS/Linux file systems and avoids emoji/RTL/zero-width
-// surprises in subject lines.
-const SAFE_CHARS = /[^A-Za-z0-9 _\-().,!@#&+=[\]{}']/g;
+// Allow any Unicode letter or digit (so umlauts, accents, CJK survive) plus a
+// small set of safe punctuation. Everything else - emojis, RTL/zero-width
+// marks, control chars, and the filesystem-reserved `<>:"/\|?*` - collapses
+// to `_`. Keeps filenames usable across Windows/macOS/Linux without flattening
+// non-ASCII scripts.
+const SAFE_CHARS = /[^\p{L}\p{N} _\-().,!@#&+=[\]{}']/gu;
 
 export const DEFAULT_EMAIL_TEMPLATE = "{date} ({from}-{to}) {subject}";
 export const DEFAULT_ATTACHMENT_TEMPLATE = "{filename}";