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diff --git a/testing/web-platform/tests/FileAPI/support/send-file-formdata-helper.js b/testing/web-platform/tests/FileAPI/support/send-file-formdata-helper.js
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+++ b/testing/web-platform/tests/FileAPI/support/send-file-formdata-helper.js
@@ -0,0 +1,99 @@
+"use strict";
+
+const kTestChars = "ABC~‾¥≈¤･・•∙·☼★星🌟星★☼·∙•・･¤≈¥‾~XYZ";
+
+// formDataPostFileUploadTest - verifies multipart upload structure and
+// numeric character reference replacement for filenames, field names,
+// and field values using FormData and fetch().
+//
+// Uses /fetch/api/resources/echo-content.py to echo the upload
+// POST (unlike in send-file-form-helper.js, here we expect all
+// multipart/form-data request bodies to be UTF-8, so we don't need to
+// escape controls and non-ASCII bytes).
+//
+// Fields in the parameter object:
+//
+// - fileNameSource: purely explanatory and gives a clue about which
+//   character encoding is the source for the non-7-bit-ASCII parts of
+//   the fileBaseName, or Unicode if no smaller-than-Unicode source
+//   contains all the characters. Used in the test name.
+// - fileBaseName: the not-necessarily-just-7-bit-ASCII file basename
+//   used for the constructed test file. Used in the test name.
+const formDataPostFileUploadTest = ({
+  fileNameSource,
+  fileBaseName,
+}) => {
+  promise_test(async (testCase) => {
+    const formData = new FormData();
+    let file = new Blob([kTestChars], { type: "text/plain" });
+    try {
+      // Switch to File in browsers that allow this
+      file = new File([file], fileBaseName, { type: file.type });
+    } catch (ignoredException) {
+    }
+
+    // Used to verify that the browser agrees with the test about
+    // field value replacement and encoding independently of file system
+    // idiosyncracies.
+    formData.append("filename", fileBaseName);
+
+    // Same, but with name and value reversed to ensure field names
+    // get the same treatment.
+    formData.append(fileBaseName, "filename");
+
+    formData.append("file", file, fileBaseName);
+
+    const formDataText = await (await fetch(
+      `/fetch/api/resources/echo-content.py`,
+      {
+        method: "POST",
+        body: formData,
+      },
+    )).text();
+    const formDataLines = formDataText.split("\r\n");
+    if (formDataLines.length && !formDataLines[formDataLines.length - 1]) {
+      --formDataLines.length;
+    }
+    assert_greater_than(
+      formDataLines.length,
+      2,
+      `${fileBaseName}: multipart form data must have at least 3 lines: ${
+        JSON.stringify(formDataText)
+      }`,
+    );
+    const boundary = formDataLines[0];
+    assert_equals(
+      formDataLines[formDataLines.length - 1],
+      boundary + "--",
+      `${fileBaseName}: multipart form data must end with ${boundary}--: ${
+        JSON.stringify(formDataText)
+      }`,
+    );
+
+    const asValue = fileBaseName.replace(/\r\n?|\n/g, "\r\n");
+    const asName = asValue.replace(/[\r\n"]/g, encodeURIComponent);
+    const asFilename = fileBaseName.replace(/[\r\n"]/g, encodeURIComponent);
+    const expectedText = [
+      boundary,
+      'Content-Disposition: form-data; name="filename"',
+      "",
+      asValue,
+      boundary,
+      `Content-Disposition: form-data; name="${asName}"`,
+      "",
+      "filename",
+      boundary,
+      `Content-Disposition: form-data; name="file"; ` +
+      `filename="${asFilename}"`,
+      "Content-Type: text/plain",
+      "",
+      kTestChars,
+      boundary + "--",
+    ].join("\r\n");
+
+    assert_true(
+      formDataText.startsWith(expectedText),
+      `Unexpected multipart-shaped form data received:\n${formDataText}\nExpected:\n${expectedText}`,
+    );
+  }, `Upload ${fileBaseName} (${fileNameSource}) in fetch with FormData`);
+};