Finalize APIv2 for attachments
Finalize APIv2 for attachments

APIv2 handling of attachments has many improvements:
* rigorous access control on all methods
* inline display of downloaded files
* storage of MIME content type of uploaded files
* more robust error checking of chucked uploads
* ability to provide content in both the POST and PUT requests of a chunked
  upload, reducing the total number of required requests by 1
* much more efficient memory usage when uploading large non-chunked files
* efficient support for HTTP HEAD method for attachment metadata
* better support for HTTP caching for downloaded files

SlideAtlas/SlideAtlas-Server,SlideAtlas/SlideAtlas-Server,SlideAtlas/SlideAtlas-Server,SlideAtlas/SlideAtlas-Server