The most basic File implementation.

Almost silly as-is, but handles this case:

```
if (input instanceof File) {
   throw Error('file not supported')
}
```

as seen on reddit.

src/browser/env.zig

@@ -34,6 +34,7 @@ const WebApis = struct {
         @import("url/url.zig").Interfaces,
         @import("xhr/xhr.zig").Interfaces,
         @import("xhr/form_data.zig").Interfaces,
+        @import("xhr/File.zig"),
         @import("xmlserializer/xmlserializer.zig").Interfaces,
     });
 };

src/browser/xhr/File.zig

@@ -0,0 +1,40 @@
+// Copyright (C) 2023-2025  Lightpanda (Selecy SAS)
+//
+// Francis Bouvier <francis@lightpanda.io>
+// Pierre Tachoire <pierre@lightpanda.io>
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as
+// published by the Free Software Foundation, either version 3 of the
+// License, or (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+const std = @import("std");
+
+// https://w3c.github.io/FileAPI/#file-section
+const File = @This();
+
+// Very incomplete. The prototype for this is Blob, which we don't have.
+// This minimum "implementation" is added because some JavaScript code just
+// checks: if (x instanceof File) throw Error(...)
+pub fn constructor() File {
+    return .{};
+}
+
+const testing = @import("../../testing.zig");
+test "Browser.File" {
+    var runner = try testing.jsRunner(testing.tracking_allocator, .{ .html = "" });
+    defer runner.deinit();
+
+    try runner.testCases(&.{
+        .{ "let f = new File()", null },
+        .{ "f instanceof File", "true" },
+    }, .{});
+}