Here is one of my helper methods I use to get a list of available attachments on a slot (for the current skin).

///////////////////////////////////////
// \brief       Returns a list of attachments (sprites) available for a slot position.
// \details     E.g. returns {"spear", "dagger"} for "left hand item" slot
///////////////////////////////////////
bool Animation::SpineMovie::GetSlotAttachmentList( const std::string& theSlotName, std::vector<std::string>& theReturnList )
{
   if(pSkeleton && pSkeleton->skin)
   {
      int aSlotIndex = spSkeleton_findSlotIndex(pSkeleton, theSlotName.c_str());

  if(aSlotIndex >= 0)
  {
     theReturnList.clear();

     int anAttachmentIndex = 0;
     const char* anAttachmentName = spSkin_getAttachmentName(pSkeleton->skin, aSlotIndex, anAttachmentIndex);
     while(anAttachmentName != 0)
     {
        theReturnList.push_back(anAttachmentName);
        anAttachmentName = spSkin_getAttachmentName(pSkeleton->skin, aSlotIndex, ++anAttachmentIndex);
     }

     return true;
  }
   }

   return false;
}

I'm not seeing anyway to get the attachment from the slot/attachment index, spSkin_getAttachment() requires the char* name of the attachment. Once you have the list of attachment names for the slot, you can iterate over it using spSkin_getAttachment/b.

the spAttachment* has an spAttachmentType field, which you can switch() and cast to the concrete type..

switch(attachment->type){
case SP_ATTACHMENT_REGION: // textured quad
   {
      spRegionAttachment* regionAttachment = (spRegionAttachment*)attachment; // 4 verts
   }
   break;
case SP_ATTACHMENT_BOUNDING_BOX:
    // 0
   continue;
case SP_ATTACHMENT_MESH: // textured mesh
   {
      spMeshAttachment* mesh = (spMeshAttachment*)attachment; // mesh->verticesCount
   }
   break;
case SP_ATTACHMENT_SKINNED_MESH:
   {
      spSkinnedMeshAttachment* mesh = (spSkinnedMeshAttachment*)attachment; // mesh->trianglesCount 
   }
   break;
default: 
   break;
}