Index: /trunk/src/org/openstreetmap/josm/tools/HttpClient.java
===================================================================
--- /trunk/src/org/openstreetmap/josm/tools/HttpClient.java	(revision 15967)
+++ /trunk/src/org/openstreetmap/josm/tools/HttpClient.java	(revision 15968)
@@ -535,5 +535,7 @@
      */
     public static HttpClient create(URL url, String requestMethod) {
-        return factory.create(url, requestMethod);
+        return factory.create(url, requestMethod)
+                // #18812: specify `Accept=*/*` to prevent Java from adding `Accept=text/html, image/gif, image/jpeg, *; q=.2, */*; q=.2`
+                .setAccept("*/*");
     }
 
Index: /trunk/test/functional/org/openstreetmap/josm/tools/HttpClientTest.java
===================================================================
--- /trunk/test/functional/org/openstreetmap/josm/tools/HttpClientTest.java	(revision 15967)
+++ /trunk/test/functional/org/openstreetmap/josm/tools/HttpClientTest.java	(revision 15968)
@@ -85,5 +85,5 @@
         assertThat(client.getURL(), is(new URL("https://httpbin.org/")));
         assertThat(client.getRequestMethod(), is("GET"));
-        assertThat(client.getRequestHeader("Accept"), nullValue());
+        assertThat(client.getRequestHeader("Accept"), is("*/*"));
         client.setAccept("text/html");
         assertThat(client.getRequestHeader("Accept"), is("text/html"));
@@ -119,4 +119,19 @@
             assertThat(root.getJsonObject("headers").get("Cache-Control"), nullValue());
             assertThat(root.getJsonObject("headers").get("Pragma"), nullValue());
+        }
+    }
+
+    /**
+     * Test JOSM User-Agent
+     * @throws IOException if an I/O error occurs
+     */
+    @Test
+    public void testHeaders() throws IOException {
+        try (InputStream in = HttpClient.create(new URL("https://httpbin.org/headers")).connect(progress).getContent();
+             JsonReader json = JsonProvider.provider().createReader(in)) {
+            final JsonObject headers = json.readObject().getJsonObject("headers");
+            assertThat(headers.getString("Accept"), is("*/*"));
+            assertThat(headers.getString("Accept-Encoding"), is("gzip, deflate"));
+            assertThat(headers.getString("User-Agent"), is(Version.getInstance().getFullAgentString()));
         }
     }