On Issue Navigator full content view, the FileSize bean is still referenced as com.atlassian.jira.web.util.FileSize. The bean has moved to Atlassian Core and should be referenced as com.atlassian.core.util.FileSize

To fix the problem change /secure/views/navigator/navigator-full.jsp from:
<webwork:bean name="'com.atlassian.jira.web.util.FileSize'" id="sizeFormatter" />

to:
<webwork:bean name="'com.atlassian.core.util.FileSize'" id="sizeFormatter" />