Pierre-Yves David <pierre-yves.david@fb.com> [Thu, 18 Dec 2014 19:14:01 -0800] rev 23868
bundle2: enforce parttype as alphanumerical
The binary format description has always stated that the parttype should be simple,
but it was never really enforced. Recent discussions have convinced me we want to
keep the part type simple and easy to debug. There is enough extensibility in
the rest of the format.