Avoid substr() error when strpos returns false

"Exception: substr() expects parameter 3 to be int, bool given" can occur on Line 378 $mimePart = substr($icon, 0, strpos($icon, '-'));
This happens, when '-' is not found and strpos returns false instead of an int.
When this occurs, e.g., Activity hangs.

Signed-off-by: lui87kw <lukas.ifflaender@uni-wuerzburg.de>