diff --git a/lib/external/s3.js b/lib/external/s3.js
index f8f2467ab..ee77ba240 100644
--- a/lib/external/s3.js
+++ b/lib/external/s3.js
@@ -118,7 +118,9 @@ const init = (config) => {
   // * https://docs.aws.amazon.com/AmazonS3/latest/API/API_GetObject.html#API_GetObject_RequestSyntax
   const getRespHeaders = (filename, { contentType }) => ({
     'response-content-disposition': contentDisposition(filename),
-    'response-content-type': contentType,
+    // "null" is a questionable content-type, but matches current central behaviour
+    // See: https://github.com/getodk/central-backend/pull/1352
+    'response-content-type': contentType || 'null',
   });
 
   async function urlForBlob(filename, blob) {
diff --git a/test/e2e/s3/test-forms/1-attachments/cities.geojson b/test/e2e/s3/test-forms/1-attachments/cities.geojson
new file mode 100644
index 000000000..0967ef424
--- /dev/null
+++ b/test/e2e/s3/test-forms/1-attachments/cities.geojson
@@ -0,0 +1 @@
+{}
diff --git a/test/e2e/s3/test-forms/1-attachments/some.nomime b/test/e2e/s3/test-forms/1-attachments/some.nomime
new file mode 100644
index 000000000..5a70c362b
--- /dev/null
+++ b/test/e2e/s3/test-forms/1-attachments/some.nomime
@@ -0,0 +1,2 @@
+I should not be uploaded with a mime type.
+TODO what mime type should I be DOWNLOADED with?
diff --git a/test/e2e/s3/test-forms/1.xml b/test/e2e/s3/test-forms/1.xml
index 73e0e9bbb..668bf3d1b 100644
--- a/test/e2e/s3/test-forms/1.xml
+++ b/test/e2e/s3/test-forms/1.xml
@@ -49,6 +49,14 @@
             <value>Smile</value>
             <value form="image">jr://images/smile.png</value>
           </text>
+          <text id="geojson">
+            <value>geojson</value>
+            <value form="image">jr://images/cities.geojson</value>
+          </text>
+          <text id="nomime">
+            <value>geojson</value>
+            <value form="image">jr://images/some.nomime</value>
+          </text>
         </translation>
       </itext>
       <instance>
diff --git a/test/e2e/s3/test.js b/test/e2e/s3/test.js
index 26fd2f84c..b94c2e75c 100644
--- a/test/e2e/s3/test.js
+++ b/test/e2e/s3/test.js
@@ -90,13 +90,13 @@ describe('s3 support', () => {
 
     // given
     await setup(1);
-    await assertNewStatuses({ pending: 11 });
+    await assertNewStatuses({ pending: 13 });
 
     // when
     await cli('upload-pending');
 
     // then
-    await assertNewStatuses({ uploaded: 11 });
+    await assertNewStatuses({ uploaded: 13 });
     // and
     await assertAllRedirect(actualAttachments);
     await assertAllDownloadsMatchOriginal(actualAttachments);
@@ -379,7 +379,10 @@ describe('s3 support', () => {
 
     const filepath = `${attDir}/${name}`;
 
-    const expectedContentType = mimetypeFor(name);
+    // "null" is a questionable content-type, but matches current central behaviour
+    // See: https://github.com/getodk/central-backend/pull/1352
+    const expectedContentType = mimetypeFor(name) ?? 'null';
+
     const actualContentType = res.headers.get('content-type');
     should.equal(actualContentType, expectedContentType);
 
diff --git a/test/e2e/util/api.js b/test/e2e/util/api.js
index 50773e355..1c9c5c1f4 100644
--- a/test/e2e/util/api.js
+++ b/test/e2e/util/api.js
@@ -46,7 +46,11 @@ async function apiClient(suiteName, { serverUrl, userEmail, userPassword, logPat
       });
     } else {
       const { body, mimeType } = opts;
-      return apiPost(path, body, { 'Content-Type':mimeType });
+
+      const headers = {};
+      if(mimeType) headers['Content-Type'] = mimeType;
+
+      return apiPost(path, body, headers);
     }
   }
 
@@ -119,14 +123,16 @@ function mimetypeFor(f) {
   // For more, see: https://developer.mozilla.org/en-US/docs/Web/HTTP/Basics_of_HTTP/MIME_types/Common_types
   const extension = extname(f);
   switch(extension) {
-    case '.bin' : return 'application/octet-stream';
-    case '.jpg' : return 'image/jpeg';
-    case '.png' : return 'image/png';
-    case '.svg' : return 'image/svg+xml';
-    case '.txt' : return 'text/plain';
-    case '.xls' : return 'application/vnd.ms-excel';
-    case '.xlsx': return 'application/vnd.openxmlformats-officedocument.spreadsheetml.sheet';
-    case '.xml' : return 'application/xml';
+    case '.bin'    : return 'application/octet-stream';
+    case '.geojson': return 'application/geo+json';
+    case '.jpg'    : return 'image/jpeg';
+    case '.nomime' : return null; // used for testing user agents which do not set a mime type for some file extensions
+    case '.png'    : return 'image/png';
+    case '.svg'    : return 'image/svg+xml';
+    case '.txt'    : return 'text/plain';
+    case '.xls'    : return 'application/vnd.ms-excel';
+    case '.xlsx'   : return 'application/vnd.openxmlformats-officedocument.spreadsheetml.sheet';
+    case '.xml'    : return 'application/xml';
     default: throw new Error(`Unsure what mime type to use for: ${f}`);
   }
 }