harness: attachments named by real m.filename, not the caption; mimetype-derived extension fallback

src/matrix.zig

@@ -339,6 +339,8 @@ pub const Event = struct {
     url: ?[]u8 = null, // allocated; caller frees
     mimetype: []u8 = "", // allocated; caller frees
     size: i64 = 0,
+    /// Original attachment filename (distinct from body, which is the caption).
+    filename: []u8 = &.{}, // allocated; caller frees
 };
 
 /// One sync poll. Returns message events + next `since` token. Caller frees.
@@ -438,6 +440,11 @@ pub fn sync(alloc: std.mem.Allocator, io: Io, token: []const u8, since: []const
 
             // Attachment fields (m.file / m.image / m.audio / m.video).
             var url_copy: ?[]u8 = null;
+            var fname_copy: []u8 = "";
+            if (content.get("filename")) |f| switch (f) {
+                .string => |v| fname_copy = alloc.dupe(u8, v) catch "",
+                else => {},
+            };
             if (content.get("url")) |u| switch (u) {
                 .string => |v| url_copy = alloc.dupe(u8, v) catch null,
                 else => {},
@@ -463,7 +470,7 @@ pub fn sync(alloc: std.mem.Allocator, io: Io, token: []const u8, since: []const
                 alloc.free(sender_copy);
                 continue;
             };
-            events.append(alloc, .{ .sender = sender_copy, .body = body_copy, .ts = ts, .url = url_copy, .mimetype = mime, .size = size }) catch {
+            events.append(alloc, .{ .sender = sender_copy, .body = body_copy, .ts = ts, .url = url_copy, .filename = fname_copy, .mimetype = mime, .size = size }) catch {
                 alloc.free(sender_copy);
                 alloc.free(body_copy);
             };