To debug, I added the following debug code:

[2] ITKv4 ((c313377...) )$ git diff
diff --git a/Modules/IO/ImageBase/src/itkImageIOFactory.cxx b/Modules/IO/ImageBase/src/itkImageIOFactory.cxx
index 4f7a101..edb3a2d 100644
--- a/Modules/IO/ImageBase/src/itkImageIOFactory.cxx
+++ b/Modules/IO/ImageBase/src/itkImageIOFactory.cxx
@@ -27,10 +27,16 @@ ImageIOFactory::CreateImageIO(const char path, FileModeType mode)
std::list< ImageIOBase::Pointer > possibleImageIO;
std::list< LightObject::Pointer > allobjects =
ObjectFactoryBase::CreateAllInstance("itkImageIOBase");

• std::cerr << "Classes: "
• << allobjects.size()
• << std::endl;
  for ( std::list< LightObject::Pointer >::iterator i = allobjects.begin();
  i != allobjects.end(); ++i )
  {
  ImageIOBase io = dynamic_cast< ImageIOBase >( i->GetPointer() );
• std::cerr << "Testing: "
• << ( *i )->GetNameOfClass()
• << std::endl;
  if ( io )
  {
  possibleImageIO.push_back(io);

Then after trying to load a volume I get this in the error log:

Classes: 4
Testing: MGHImageIO
Testing: MRMLIDImageIO
Testing: AnalyzeImageIO
Testing: GE5ImageIO

So it appears that the ImageIO classes that slicer explicitly adds are available, but not the ITK default ones.

This code is to get you started:

https://github.com/Slicer/Slicer/tree/master/Libs/ITKFactoryRegistration

Fixed in r21652
See http://viewvc.slicer.org/viewvc.cgi/Slicer4?view=revision&amp;revision=21652

Closing resolved issues that have not been updated in more than 3 months

Fix committed to 2145-support-for-installing-extension-from-file branch.