[Bugfix] Emit a valid media type from encode_{audio,image,video}_url

[vineethsaivs]

Purpose

encode_audio_url, encode_image_url and encode_video_url in vllm/multimodal/utils.py build a data URL and resolve its media type with:

mimetype = mimetypes.types_map.get("." + format.lower(), "image")  # or "audio"/"video"
return f"data:{mimetype};base64,{...}"

When the format's extension is not registered in mimetypes.types_map, the fallback is a bare top-level type ("image", "audio", "video"). A media type with no subtype is not well-formed, so formats missing from the table produce an invalid data URL such as data:image;base64,.... This is reachable with formats Pillow/soundfile can encode but that Python's mimetypes does not register, for example TGA/PCX images or CAF/W64 audio.

This changes the fallback to "<kind>/<format>" (e.g. image/tga), so the media type is always a well-formed type/subtype. Registered formats are unchanged (e.g. PNG still resolves to image/png).

This only affects the media-type string of the generated data URL (a request-construction helper); it does not touch model output, accuracy, or serving numerics, so no model-evaluation results apply.

Not a duplicate: I searched open PRs for encode_image_url/encode_audio_url, mimetype, and multimodal/utils.py; the open multimodal PRs (#43638, #46947, #44474, #29034, ...) touch examples, spec-decode embeddings, and video URL loading, none change the encode_*_url media-type fallback. No open issue tracks this.

Test Plan

Added tests/multimodal/test_utils.py::test_encode_image_url_uncommon_format_has_valid_mimetype, which encodes a Pillow image with format="TGA" (.tga is not in mimetypes.types_map) and asserts the media type is image/tga:

pytest tests/multimodal/test_utils.py::test_encode_image_url_uncommon_format_has_valid_mimetype -q

Test Result

A full CUDA build was not available on my machine, so I validated the exact fixed function bodies on CPU by extracting encode_{audio,image,video}_url from the module and executing them with the base64 encoders stubbed (the media-type logic is independent of encoding). The added test runs in CI.

• Before, uncommon formats produced bare types: encode_image_url(img, format="TGA") -> data:image;base64,..., encode_video_url(frames, format="MKV") -> data:video;base64,....
• After: data:image/tga;base64,..., data:audio/caf;base64,..., data:video/mkv;base64,... (all well-formed type/subtype).
• Registered formats unchanged: PNG -> image/png, WAV -> audio/x-wav.

Ran ruff check and ruff format --check on both changed files: clean.

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AI assistance disclosure: AI assistance was used to help identify and prepare this change, per the repository's contribution policy. The fix updates a one-line fallback in three sibling helpers plus a regression test, and I have reviewed every changed line.

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