When JIRA is run under OpenJDK and a .png image attached to an issue, the thumbnail breaks with a stacktrace:

ERROR [core.util.thumbnail.Thumber] Unable to create thumbnail image of attachment with id:10000
java.lang.IncompatibleClassChangeError: Found class com.sun.image.codec.jpeg.JPEGImageEncoder, but interface was expected
at com.atlassian.core.util.thumbnail.Thumber.storeThumbImage(Thumber.java:265)
at com.atlassian.core.util.thumbnail.Thumber.createThumbnail(Thumber.java:175)
at com.atlassian.core.util.thumbnail.Thumber.retrieveOrCreateThumbNail(Thumber.java:131)
at com.atlassian.core.util.thumbnail.Thumber.retrieveOrCreateThumbNail(Thumber.java:91)
at com.atlassian.jira.issue.thumbnail.DefaultThumbnailManager.getThumbnail(DefaultThumbnailManager.java:88)
at com.atlassian.jira.issue.thumbnail.DefaultThumbnailManager.getThumbnails(DefaultThumbnailManager.java:50)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
....

From the equivalent Confluence issue, CONF-14791, it looks like the fix is relatively straightforward, replacing com.sun.* calls with javax.imageio.*.