LibWeb+LibWebView+WebContent: Implement more <input type=file> behavior

We had previous implemented some plumbing for file input elements in
commit 636602a54e.

This implements the return path for chromes to inform WebContent of the
file(s) the user selected. This patch includes a dummy implementation
for headless-browser to enable testing.

Userland/Libraries/LibWeb/HTML/SelectedFile.cpp

@@ -0,0 +1,70 @@
+/*
+ * Copyright (c) 2024, Tim Flynn <trflynn89@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/LexicalPath.h>
+#include <LibCore/File.h>
+#include <LibIPC/Decoder.h>
+#include <LibIPC/Encoder.h>
+#include <LibWeb/HTML/SelectedFile.h>
+
+namespace Web::HTML {
+
+ErrorOr<SelectedFile> SelectedFile::from_file_path(ByteString const& file_path)
+{
+    // https://html.spec.whatwg.org/multipage/input.html#file-upload-state-(type=file):concept-input-file-path
+    // Filenames must not contain path components, even in the case that a user has selected an entire directory
+    // hierarchy or multiple files with the same name from different directories.
+    auto name = LexicalPath::basename(file_path);
+
+    auto file = TRY(Core::File::open(file_path, Core::File::OpenMode::Read));
+    return SelectedFile { move(name), IPC::File { *file } };
+}
+
+SelectedFile::SelectedFile(ByteString name, ByteBuffer contents)
+    : m_name(move(name))
+    , m_file_or_contents(move(contents))
+{
+}
+
+SelectedFile::SelectedFile(ByteString name, IPC::File file)
+    : m_name(move(name))
+    , m_file_or_contents(move(file))
+{
+}
+
+ByteBuffer SelectedFile::take_contents()
+{
+    VERIFY(m_file_or_contents.has<ByteBuffer>());
+    return move(m_file_or_contents.get<ByteBuffer>());
+}
+
+}
+
+template<>
+ErrorOr<void> IPC::encode(Encoder& encoder, Web::HTML::SelectedFile const& file)
+{
+    TRY(encoder.encode(file.name()));
+    TRY(encoder.encode(file.file_or_contents()));
+    return {};
+}
+
+template<>
+ErrorOr<Web::HTML::SelectedFile> IPC::decode(Decoder& decoder)
+{
+    auto name = TRY(decoder.decode<ByteString>());
+    auto file_or_contents = TRY((decoder.decode<Variant<IPC::File, ByteBuffer>>()));
+
+    ByteBuffer contents;
+
+    if (file_or_contents.has<IPC::File>()) {
+        auto file = TRY(Core::File::adopt_fd(file_or_contents.get<IPC::File>().take_fd(), Core::File::OpenMode::Read));
+        contents = TRY(file->read_until_eof());
+    } else {
+        contents = move(file_or_contents.get<ByteBuffer>());
+    }
+
+    return Web::HTML::SelectedFile { move(name), move(contents) };
+}