fix check for jpeg magic bytes (cherry picked from commit 8ee8756dc06cb409650958224f58adf0806c1942)
fix check for jpeg magic bytes
(cherry picked from commit 8ee8756dc06cb409650958224f58adf0806c1942)

Conflicts:

	mapproxy/test/image.py

vrsource/mapproxy,geoadmin/mapproxy,mapproxy/mapproxy,vrsource/mapproxy,procrastinatio/mapproxy,drnextgis/mapproxy,faegi/mapproxy,mapproxy/mapproxy,geoadmin/mapproxy,olt/mapproxy,faegi/mapproxy,procrastinatio/mapproxy,kaiCu/mapproxy,olt/mapproxy,drnextgis/mapproxy,camptocamp/mapproxy,kaiCu/mapproxy,camptocamp/mapproxy