Move code handling filename into its own templage
Remove test and generation of "original filename: " prefix.

In previous versions of the metadata, this prefix was not always
there, and it would be create by the XSLT stylesheet. However, the
metadata has been updated, and now the prefix string should always
be present.
Remove comment
Customize handling of filename for Frances Perkins

Some filenames in Omeka are prefixed with the string "perkins_".
Customized code handles this, as well as the usual replace .jpg
with .tif .