Try image as ELF file first. (Don't yet actually try to execute it).

src/tests/tftptest.c

 #include <gpxe/async.h>
 #include <gpxe/uaccess.h>
 #include <gpxe/buffer.h>
+#include <gpxe/elf.h>
+#include <bios.h>
 #include "pxe.h"
 
 int test_tftp ( struct net_device *netdev, struct sockaddr_tcpip *target,
 		const char *filename ) {
 	struct tftp_session tftp;
 	struct buffer buffer;
+	struct elf elf;
+	uint16_t fbms;
 	int rc;
 
 	memset ( &buffer, 0, sizeof ( buffer ) );
 	buffer.addr = real_to_user ( 0, 0x7c00 );
-	buffer.len = ( 512 * 1024 - 0x7c00 );
+	get_real ( fbms, BDA_SEG, BDA_FBMS );
+	buffer.len = ( fbms * 1024 - 0x7c00 );
 
 	memset ( &tftp, 0, sizeof ( tftp ) );
 	udp_connect ( &tftp.udp, target );
 	if ( ( rc = async_wait ( tftp_get ( &tftp ) ) ) != 0 )
 		return rc;
 
+	elf.image = buffer.addr;
+	elf.len = buffer.len;
+	if ( ( rc = elf_load ( &elf ) ) == 0 ) {
+		printf ( "Got valid ELF image: execaddr at %lx\n",
+			 elf.entry );
+		return 0;
+	}
+
 	printf ( "Attempting PXE boot\n" );
 	pxe_netdev = netdev;
 	rc = pxe_boot();