Freeradius Dhcp and Tftp Pxe.

I have Freeradius 3.0.19 acting as AAA and Dhcp, there is also atftpd with iPXE bootloader for PXE booting. Such clients are defined by %{string:&DHCP-Vendor-Class-Identifier} and should get address from IP Pool from flat text file. Other clients (for example Windows) receive IP successfuly. But PXE clients don' t get IP at all. Why ? If address assignment is set up via static mac-ip bindings (mac2ip file) IP is assigned but boot file undionly.pxe is not found. Because Freeradius adds "M-^?" (in ASCII without quotes) or in 0x4d2d5e3fff between file name undionly.pxe and trailing zero. Why ? And how do it correct ?