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.
I agree, this is confusing code. However, it turns out that this property is only called on Windows. My suggestion would be to refactor the property/method names to something like CommandToLaunchBloom_Windows or at least add a comment.