Add extension traitlet to force dotted names
Import extension traitlet in top-level nbconvert
Rename Extension to FilenameExtension
Use cast_bytes_py2
More descriptive error messages
Cast to unicode, not bytes
Inherit from Unicode traitlet
Remove redundant check
Handle case where value is an empty string
Merge pull request #7181 from jhamrick/extension-traitlet

Add extension traitlet to force dotted names