image files with different extensions should generate thumbnails with di...

...stinct names (TNL-532)

@cpennington 
@zubair-arbi 
You could capture the extension with `root, ext = os.path.splitext(original_name)`, and then compare `ext` and `XASSET_THUMBNAIL_TAIL_NAME` directly, rather than using `endswith`. Also, you're doing `replace('.', '-')` twice, which seems redundant.