diff --git a/ext/fetch/23_request.js b/ext/fetch/23_request.js
index 1ffdd2a60d..9672287df3 100644
--- a/ext/fetch/23_request.js
+++ b/ext/fetch/23_request.js
@@ -18,7 +18,7 @@
   const { guardFromHeaders } = window.__bootstrap.headers;
   const { mixinBody, extractBody } = window.__bootstrap.fetchBody;
   const { getLocationHref } = window.__bootstrap.location;
-  const mimesniff = window.__bootstrap.mimesniff;
+  const { extractMimeType } = window.__bootstrap.mimesniff;
   const { blobFromObjectUrl } = window.__bootstrap.file;
   const {
     headersFromHeaderList,
@@ -32,9 +32,6 @@
     ArrayPrototypeMap,
     ArrayPrototypeSlice,
     ArrayPrototypeSplice,
-    MapPrototypeHas,
-    MapPrototypeGet,
-    MapPrototypeSet,
     ObjectKeys,
     ObjectPrototypeIsPrototypeOf,
     RegExpPrototypeTest,
@@ -174,41 +171,11 @@
     /** @type {AbortSignal} */
     [_signal];
     get [_mimeType]() {
-      let charset = null;
-      let essence = null;
-      let mimeType = null;
       const values = getDecodeSplitHeader(
         headerListFromHeaders(this[_headers]),
         "Content-Type",
       );
-      if (values === null) return null;
-      for (const value of values) {
-        const temporaryMimeType = mimesniff.parseMimeType(value);
-        if (
-          temporaryMimeType === null ||
-          mimesniff.essence(temporaryMimeType) == "*/*"
-        ) {
-          continue;
-        }
-        mimeType = temporaryMimeType;
-        if (mimesniff.essence(mimeType) !== essence) {
-          charset = null;
-          const newCharset = MapPrototypeGet(mimeType.parameters, "charset");
-          if (newCharset !== undefined) {
-            charset = newCharset;
-          }
-          essence = mimesniff.essence(mimeType);
-        } else {
-          if (
-            MapPrototypeHas(mimeType.parameters, "charset") === null &&
-            charset !== null
-          ) {
-            MapPrototypeSet(mimeType.parameters, "charset", charset);
-          }
-        }
-      }
-      if (mimeType === null) return null;
-      return mimeType;
+      return extractMimeType(values);
     }
     get [_body]() {
       return this[_request].body;
diff --git a/ext/fetch/23_response.js b/ext/fetch/23_response.js
index 03f8c276b9..f1f6fe1dda 100644
--- a/ext/fetch/23_response.js
+++ b/ext/fetch/23_response.js
@@ -18,7 +18,7 @@
   const { HTTP_TAB_OR_SPACE, regexMatcher } = window.__bootstrap.infra;
   const { extractBody, mixinBody } = window.__bootstrap.fetchBody;
   const { getLocationHref } = window.__bootstrap.location;
-  const mimesniff = window.__bootstrap.mimesniff;
+  const { extractMimeType } = window.__bootstrap.mimesniff;
   const { URL } = window.__bootstrap.url;
   const {
     getDecodeSplitHeader,
@@ -30,9 +30,6 @@
   const {
     ArrayPrototypeMap,
     ArrayPrototypePush,
-    MapPrototypeHas,
-    MapPrototypeGet,
-    MapPrototypeSet,
     ObjectPrototypeIsPrototypeOf,
     RangeError,
     RegExp,
@@ -162,41 +159,11 @@
 
   class Response {
     get [_mimeType]() {
-      let charset = null;
-      let essence = null;
-      let mimeType = null;
       const values = getDecodeSplitHeader(
         headerListFromHeaders(this[_headers]),
         "Content-Type",
       );
-      if (values === null) return null;
-      for (const value of values) {
-        const temporaryMimeType = mimesniff.parseMimeType(value);
-        if (
-          temporaryMimeType === null ||
-          mimesniff.essence(temporaryMimeType) == "*/*"
-        ) {
-          continue;
-        }
-        mimeType = temporaryMimeType;
-        if (mimesniff.essence(mimeType) !== essence) {
-          charset = null;
-          const newCharset = MapPrototypeGet(mimeType.parameters, "charset");
-          if (newCharset !== undefined) {
-            charset = newCharset;
-          }
-          essence = mimesniff.essence(mimeType);
-        } else {
-          if (
-            MapPrototypeHas(mimeType.parameters, "charset") === null &&
-            charset !== null
-          ) {
-            MapPrototypeSet(mimeType.parameters, "charset", charset);
-          }
-        }
-      }
-      if (mimeType === null) return null;
-      return mimeType;
+      return extractMimeType(values);
     }
     get [_body]() {
       return this[_response].body;
diff --git a/ext/web/01_mimesniff.js b/ext/web/01_mimesniff.js
index c6ac7c66c3..d2c784d6eb 100644
--- a/ext/web/01_mimesniff.js
+++ b/ext/web/01_mimesniff.js
@@ -12,6 +12,7 @@
   const {
     ArrayPrototypeIncludes,
     Map,
+    MapPrototypeGet,
     MapPrototypeHas,
     MapPrototypeSet,
     RegExpPrototypeTest,
@@ -207,5 +208,51 @@
     return serialization;
   }
 
-  window.__bootstrap.mimesniff = { parseMimeType, essence, serializeMimeType };
+  /**
+   * Part of the Fetch spec's "extract a MIME type" algorithm
+   * (https://fetch.spec.whatwg.org/#concept-header-extract-mime-type).
+   * @param {string[] | null} headerValues The result of getting, decoding and
+   * splitting the "Content-Type" header.
+   * @returns {MimeType | null}
+   */
+  function extractMimeType(headerValues) {
+    if (headerValues === null) return null;
+
+    let charset = null;
+    let essence_ = null;
+    let mimeType = null;
+    for (const value of headerValues) {
+      const temporaryMimeType = parseMimeType(value);
+      if (
+        temporaryMimeType === null ||
+        essence(temporaryMimeType) == "*/*"
+      ) {
+        continue;
+      }
+      mimeType = temporaryMimeType;
+      if (essence(mimeType) !== essence_) {
+        charset = null;
+        const newCharset = MapPrototypeGet(mimeType.parameters, "charset");
+        if (newCharset !== undefined) {
+          charset = newCharset;
+        }
+        essence_ = essence(mimeType);
+      } else {
+        if (
+          !MapPrototypeHas(mimeType.parameters, "charset") &&
+          charset !== null
+        ) {
+          MapPrototypeSet(mimeType.parameters, "charset", charset);
+        }
+      }
+    }
+    return mimeType;
+  }
+
+  window.__bootstrap.mimesniff = {
+    parseMimeType,
+    essence,
+    serializeMimeType,
+    extractMimeType,
+  };
 })(this);
diff --git a/ext/web/internal.d.ts b/ext/web/internal.d.ts
index 41d4c9b325..8e580495b1 100644
--- a/ext/web/internal.d.ts
+++ b/ext/web/internal.d.ts
@@ -60,6 +60,9 @@ declare namespace globalThis {
       declare function parseMimeType(input: string): MimeType | null;
       declare function essence(mimeType: MimeType): string;
       declare function serializeMimeType(mimeType: MimeType): string;
+      declare function extractMimeType(
+        headerValues: string[] | null,
+      ): MimeType | null;
     }
 
     declare var eventTarget: {