diff --git a/tests/multimodal/test_utils.py b/tests/multimodal/test_utils.py
index ffc8ce504c..97f46de13c 100644
--- a/tests/multimodal/test_utils.py
+++ b/tests/multimodal/test_utils.py
@@ -2,6 +2,7 @@
 # SPDX-FileCopyrightText: Copyright contributors to the vLLM project
 import pytest
 import torch
+from PIL import Image
 
 from vllm.multimodal.inputs import (
     MultiModalBatchedField,
@@ -10,7 +11,11 @@ from vllm.multimodal.inputs import (
     MultiModalSharedField,
     PlaceholderRange,
 )
-from vllm.multimodal.utils import argsort_mm_positions, group_and_batch_mm_items
+from vllm.multimodal.utils import (
+    argsort_mm_positions,
+    encode_image_url,
+    group_and_batch_mm_items,
+)
 
 
 @pytest.mark.parametrize(
@@ -224,6 +229,16 @@ def test_group_and_batch_mm_items_splits_shared_data_by_dtype():
     assert [num_items for num_items, _ in res] == [1, 1, 1]
 
 
+def test_encode_image_url_uncommon_format_has_valid_mimetype():
+    """A format missing from mimetypes.types_map must still produce a
+    well-formed type/subtype media type, not a bare 'image'."""
+    # ".tga" is not registered in mimetypes.types_map, so it exercises the
+    # fallback path; Pillow can still encode it.
+    url = encode_image_url(Image.new("RGB", (2, 2)), format="TGA")
+    mimetype = url.removeprefix("data:").split(";", 1)[0]
+    assert mimetype == "image/tga"
+
+
 def test_group_and_batch_mm_items_split_by_shared_data():
     elem1 = MultiModalFieldElem(
         data=torch.zeros(1, dtype=torch.uint8),
diff --git a/vllm/multimodal/utils.py b/vllm/multimodal/utils.py
index 25c811fd42..fc400710f0 100644
--- a/vllm/multimodal/utils.py
+++ b/vllm/multimodal/utils.py
@@ -57,7 +57,7 @@ def encode_audio_url(
 ) -> str:
     """Encode audio as a data URL."""
     audio_b64 = encode_audio_base64(audio, sampling_rate, format=format)
-    mimetype = mimetypes.types_map.get("." + format.lower(), "audio")
+    mimetype = mimetypes.types_map.get("." + format.lower(), f"audio/{format.lower()}")
     return f"data:{mimetype};base64,{audio_b64}"
 
 
@@ -90,7 +90,7 @@ def encode_image_url(
     Pass `image_mode=None` to keep the original image mode.
     """
     image_b64 = encode_image_base64(image, image_mode=image_mode, format=format)
-    mimetype = mimetypes.types_map.get("." + format.lower(), "image")
+    mimetype = mimetypes.types_map.get("." + format.lower(), f"image/{format.lower()}")
     return f"data:{mimetype};base64,{image_b64}"
 
 
@@ -114,7 +114,9 @@ def encode_video_url(
     if format.lower() == "jpeg":
         mimetype = "video/jpeg"
     else:
-        mimetype = mimetypes.types_map.get("." + format.lower(), "video")
+        mimetype = mimetypes.types_map.get(
+            "." + format.lower(), f"video/{format.lower()}"
+        )
 
     return f"data:{mimetype};base64,{video_b64}"