Websphere WAS 6.1/7 often logs the following exceptions into SystemOut.log files.

[Servlet Error]-[Filter [DeclaredResourcesPluginFilter]: filter is unavailable.]: java.lang.NullPointerException
    at com.ibm.ws.webcontainer.srt.SRTServletResponse.setContentType(SRTServletResponse.java:1141)


Solution:
1. Log into Admin console of Websphere Apllication Server.
2. Navigate to

WAS Admin -> Environments -> Virtual Hosts -> default_host -> MIME Types

Add/Modify following as the MIME TYPE 

MIME Type	Extensions
image/gif	GIF gif
image/jpeg	JPE JPEG JPG jpe jpeg jpg
image/tiff	TIFF TIF tiff tif
image/png	PNG png
image/psd	PSD psd
image/icon	ICON ICO icon ico

Note: The Extensions can be separated by whitespace


Do it in all Virtual Hosts.

See the below image for reference.