fix(mime-bytes): detect HEIC/HEIF instead of misidentifying as MP4

[pyramation]

Summary

Fixes #967. HEIC and HEIF images were misidentified as video/mp4 because all three formats share the ISO Base Media File Format ftyp container prefix. The mp4 entry (4 magic bytes) appeared before heic/heif (8 magic bytes) in the registry, so the shorter prefix matched first.

The fix sorts this.fileTypes by magicBytes.length descending in the FileTypeDetector constructor, ensuring longer (more specific) signatures are checked before shorter prefix patterns.

Review & Testing Checklist for Human

• Verify with a real .heic photo from an iPhone that detection returns image/heic instead of video/mp4
• Confirm that actual MP4 video files (e.g. with isom brand) still detect correctly as video/mp4

Notes

• The sort is stable (JS spec), so entries with identical magicBytes.length retain their original order.
• Tests added for both HEIC (ftypheic brand) and HEIF (ftypmif1 brand) buffers, plus the existing MP4 (ftypisom) test continues to pass.

Link to Devin session: https://app.devin.ai/sessions/1db29f3c8a46434c99001ddcb701cc12
Requested by: @pyramation

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