Don't lowercase filenames on Linux before opening the file

This fixes https://jira.sil.org/browse/BL-828 "[Linux] Vaccination Shell
has lost the License info for Front Cover Image".  This inspired me to
go looking for similar problems elsewhere in the Bloom code.  The actual
fix for the code is in ImageUpdater.cs.
My bad here.  These entries should match the actual filenames which are indeed camel case (except for editororiginalmode.css which doesn't seem to exist but which is referred to in the code using camel case elsewhere.  Thanks for catching this, even if somewhat inadvertantly.  :-)