Fix mimetype_from_path returning empty string when puremagic cannot detect type

[mahsumaktas]

Fixes #1340.

Problem

puremagic.from_file() can return an empty string '' (instead of raising PureError) when it cannot identify a file type. The previous code only handled the exception case, so the empty string passed through as the MIME type. This produced errors like:

Error: This model does not support attachments of type '', only audio/flac, image/heif, ...

Fix

Added a mimetypes.guess_type() fallback in mimetype_from_path() for both cases: empty string return and PureError. This covers common extensions (e.g. .mp4, .wav, .pdf) when magic number detection fails.

Applied the same empty-string guard to mimetype_from_string() so it returns None instead of '' when puremagic comes up empty (no path available there for a fallback).

Changes

llm/utils.py (5 lines changed):

• Added import mimetypes
• mimetype_from_path: returns mimetypes.guess_type() result when puremagic returns '' or raises PureError
• mimetype_from_string: returns None when puremagic returns ''

tests/test_utils.py (65 lines added):

• 8 new tests covering both functions across puremagic success, empty string, PureError, unknown extension, and MIME_TYPE_FIXES cases

Tests

tests/test_utils.py::TestMimetypeFromPath::test_puremagic_returns_valid_type PASSED
tests/test_utils.py::TestMimetypeFromPath::test_puremagic_returns_empty_string_falls_back_to_mimetypes PASSED
tests/test_utils.py::TestMimetypeFromPath::test_puremagic_raises_falls_back_to_mimetypes PASSED
tests/test_utils.py::TestMimetypeFromPath::test_unknown_extension_returns_none PASSED
tests/test_utils.py::TestMimetypeFromPath::test_mime_type_fix_applied PASSED
tests/test_utils.py::TestMimetypeFromString::test_puremagic_returns_valid_type PASSED
tests/test_utils.py::TestMimetypeFromString::test_puremagic_returns_empty_string_returns_none PASSED
tests/test_utils.py::TestMimetypeFromString::test_puremagic_raises_returns_none PASSED

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