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The .EXL files created by the Korg X3 is composed of 3 pieces.
1 - Korg machine ID             (5 bytes)
2 - File type                   (2 bytes)
3 - 00 pad area (reserved?)     (9 bytes)
4 - the lenth of the sysex file (2 bytes)
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                Total of header 18 bytes

5 - sysex file in standard sysex data format. (starts with F0 and ends with F7)

There is NOTHING else attached to the file. (i.e. no footer)

The first 16 bytes is the header and 00 pading (presumably for some other
feature later - Korg is notorious for 'reserving' bytes)  - and the lenth
is 2 bytes.

Here is the first 16 bytes in hex. (Never Changes)
4B 4F 52 47 35 00 02 00 00 00 00 00 00 00 00 00

the 17th and 18th bytes are the size of the file in hex.
17 - Low byte
18 - high byte

so if the sysex data bytes totaled 43981 bytes (or HEX ABCD ) then 
byte 17 = CD
byte 18 = AB 

Then the sysex data follows unmodified.

A map of the file structure is as follows
(byte 1 equqls the first byte in the record - not ofset value)
01-05 Korg Machine ID "KORG5"
06 - pad HEX 00
07 - File type ID (HEX 02 - System Exclusive Data)
08 - 16 Pad Hex 00
17 Sysex File Size  Low Byte
18 Sysex File Size High Byte
19-end - Sysex data (unmodified)