Fix MIME detection fallback when puremagic returns empty string

[tgies]

Summary

• Fixes MIME type detection fails when puremagic returns empty string #1340 — puremagic.from_file() can return an empty string "" instead of raising PureError, which passed through as an invalid MIME type causing attachment validation errors.
• Adds mimetypes.guess_type() as a fallback when puremagic returns empty string or raises an exception.
• Adds "audio/x-wav": "audio/wav" to MIME_TYPE_FIXES since mimetypes returns audio/x-wav for .wav files.
• Applies MIME_TYPE_FIXES normalization regardless of which detection method succeeded.

Test plan

• test_mimetype_from_path_fallback_on_empty_string — puremagic returns "", falls back to extension
• test_mimetype_from_path_fallback_on_puremagic_error — puremagic raises, falls back to extension
• test_mimetype_from_path_fallback_normalizes_wav — fallback result goes through MIME_TYPE_FIXES
• test_mimetype_from_path_returns_none_if_no_detection — both methods fail → None