fix(fetch): Correctly decode `multipart/form-data` names and filenames (#19145)

Currently the `multipart/form-data` parser in
`Request.prototype.formData` and `Response.prototype.formData` decodes
non-ASCII filenames incorrectly, as if they were encoded in Latin-1
rather than UTF-8. This happens because the header section of each
`multipart/form-data` entry is decoded as Latin-1 in order to be parsed
with `Headers`, which only allows `ByteString`s, but the names and
filenames are never decoded correctly. This PR fixes this as a
post-processing step.

Note that the `multipart/form-data` parsing for this APIs in the Fetch
spec is very much underspecified, and it does not specify that names and
filenames must be decoded as UTF-8. However, it does require that the
bodies of non-`File` entries are decoded as UTF-8, and in browsers,
names and filenames always use the same encoding as the body.

Closes #19142.

1 files changed, 53 insertions, 0 deletions

diff --git a/cli/tests/unit/body_test.ts b/cli/tests/unit/body_test.ts
index e7a38b7a6..8aebfadd3 100644
--- a/cli/tests/unit/body_test.ts
+++ b/cli/tests/unit/body_test.ts
@@ -53,6 +53,59 @@ Deno.test(
   },
 );
 
+// FormData: non-ASCII names and filenames
+Deno.test(
+  { permissions: { net: true } },
+  async function bodyMultipartFormDataNonAsciiNames() {
+    const boundary = "----01230123";
+    const payload = [
+      `--${boundary}`,
+      `Content-Disposition: form-data; name="文字"`,
+      "",
+      "文字",
+      `--${boundary}`,
+      `Content-Disposition: form-data; name="file"; filename="文字"`,
+      "Content-Type: application/octet-stream",
+      "",
+      "",
+      `--${boundary}--`,
+    ].join("\r\n");
+
+    const body = buildBody(
+      new TextEncoder().encode(payload),
+      new Headers({
+        "Content-Type": `multipart/form-data; boundary=${boundary}`,
+      }),
+    );
+
+    const formData = await body.formData();
+    assert(formData.has("文字"));
+    assertEquals(formData.get("文字"), "文字");
+    assert(formData.has("file"));
+    assert(formData.get("file") instanceof File);
+    assertEquals((formData.get("file") as File).name, "文字");
+  },
+);
+
+// FormData: non-ASCII names and filenames roundtrip
+Deno.test(
+  { permissions: { net: true } },
+  async function bodyMultipartFormDataNonAsciiRoundtrip() {
+    const inFormData = new FormData();
+    inFormData.append("文字", "文字");
+    inFormData.append("file", new File([], "文字"));
+
+    const body = buildBody(inFormData);
+
+    const formData = await body.formData();
+    assert(formData.has("文字"));
+    assertEquals(formData.get("文字"), "文字");
+    assert(formData.has("file"));
+    assert(formData.get("file") instanceof File);
+    assertEquals((formData.get("file") as File).name, "文字");
+  },
+);
+
 Deno.test(
   { permissions: { net: true } },
   async function bodyURLEncodedFormData() {