Set ramdisktftpblocksize to 8192

the default ramdisktftpblocksize is 1456 for bootmgr.exe created on a windows 2012 R2 server. The  winpe.wim file can get pretty large. In My case it was 273 MB in size.

So, in order to have a successful file transfer (at least with the Twisted implementation of tftp) the client must request a block size larger then the default.

1456 * 2^16 is 91 MB

A value of 8192 for ramdisktftpblocksize will allow file transfers of 512 MB in size.