With the move to magnolia 5 we are finding that many of our images which used to be just fine are not rendering correctly in magnolia 5.

Reason is that these images have different mime-types:
image/x-png - the mimeType used by PNG before the standard image/png was registered
image/pjpeg - mimeType used by Internet Explorer when uploading progressive jpegs

In Magnolia 4.4 these didn't cause any problems.
In Magnolia 5 the new Assets stuff can't deal with them.

The problem is (initially) with info.magnolia.module.templatingkit.dam.STKAssetRenderer
This defines a set of allowed MimeTypes, it is too restrictive.
Please add the pjpeg and x-png image types.