Werk #6439: Fixed upload of some icon files

Component	Setup
Title	Fixed upload of some icon files
Date	Aug 9, 2018
Level	Trivial Change
Class	Bug Fix
Compatibility	Compatible - no manual interaction needed
Checkmk versions & editions	1.6.0b1 Checkmk Community, Checkmk Pro, Checkmk Ultimate MT 1.5.0p1 Checkmk Community, Checkmk Pro, Checkmk Ultimate MT

The icon upload failed with some PNG images with specific meta data set.

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