Handle jpeg with COM segment

[KeremTurgutlu]

Problem Summary

Issue: imghdr.what() fails to detect valid JPEG images generated by video processing tools (like ffmpeg/libavcodec), causing _is_img() to return False even though PIL can read the images perfectly.

Root Cause: These programmatically generated JPEGs have a COM (comment) segment (0xFFFE) immediately after the SOI marker (0xFFD8), containing encoder metadata like "Lavc61.19.101". The existing test_jpeg function only checks for specific 4-byte patterns that don't include 0xFFD8FFFE.

Proposed Solution

Add b'\xff\xd8\xff\xfe' to the existing pattern list in test_jpeg.

Generalization Assessment

This change is appropriately generic and would help many cases:

1. Common pattern: Video-to-image extraction tools (ffmpeg, OpenCV, etc.) routinely add COM segments
2. Standards compliant: COM segments are valid JPEG markers per the JPEG specification
3. Low risk: Adding this pattern won't cause false positives since 0xFFFE is a reserved JPEG marker

[jph00]

Thanks @KeremTurgutlu !