[Windows Phone 8 General] styweasm:
MPRG8x26_fh.ede definitely is a hex, because in WDRT logs we have something like this:

D_MSG : C:\Program Files\Microsoft Care Suite\Windows Device Recovery Tool\thor2.exe -mode emergency -conn 0.1D07:0005:0003 -hexfile C:\ProgramData\Microsoft\Packages\Products\RM-1075\MPRG8x26_fh.ede -edfile C:\ProgramData\Microsoft\Packages\Products\RM-1075\RM1075_fh.edp -skipffuflash -disable_stdout_buffering

but when it used as hex for mbn file, we get:

[16:52:25.697] D_MSG : THOR2 1.8.2.18
[16:52:25.698] D_MSG : Built for Windows @ 13:36:46 Jun 16 2015
[16:52:25.698] D_MSG : Thor2 is running on Windows of version 6.2
[16:52:25.699] D_MSG : thor2 -mode emergency -hexfile C:\ProgramData\Microsoft\Packages\Products\RM-1075\831BE6B18E7006372069545885A80F786C33D064.hex -mbnfile C:\ProgramData\Microsoft\Packages\Products\RM-1075\msimage.mbn -orig_gpt
[16:52:25.699] D_MSG : Process started Tue Mar 01 16:52:25 2016
[16:52:25.715] D_MSG : Debugging enabled for emergency

[16:52:25.716] D_MSG : Initiating emergency download
[16:52:26.728] D_MSG : Using default emergency protocol
[16:52:26.728] D_MSG : ALPHA EMERGENCY FLASH START
[16:52:26.736] D_MSG : Emergency Programmer V1 version 2014.10.31.001
[16:52:26.737] D_MSG : Hex download selected
[16:52:26.738] D_MSG : Check if device in Dload
[16:52:48.310] D_ERR : Message send failed with error code -1
[16:52:50.311] D_MSG : Waiting connection to DLOAD: 2 of 2
[16:52:50.313] D_MSG : Check if device in Dload
[16:53:12.289] D_ERR : Message send failed with error code -1
[16:53:12.292] D_ERR : Failed to connect to DLOAD mode
[16:53:12.294] D_ERR : Make sure that the COM port is free. Close QPST.
[16:53:12.295] D_MSG : ALPHA EMERGENCY FLASH END
[16:53:17.310] D_MSG : Emergency messaging closed successfully
[16:53:17.316] D_MSG : Operation took about 52.00 seconds.
[16:53:17.324] D_ERR : THOR2 1.8.2.18 exited with error code 85030 (0x14C26)

or like this:

[17:08:57.032] D_MSG : THOR2 1.8.2.18
[17:08:57.035] D_MSG : Built for Windows @ 13:36:46 Jun 16 2015
[17:08:57.038] D_MSG : Thor2 is running on Windows of version 6.2
[17:08:57.044] D_MSG : thor2 -mode emergency -hexfile C:\ProgramData\Microsoft\Packages\Products\RM-1075\831BE6B18E7006372069545885A80F786C33D064.hex -mbnfile C:\ProgramData\Microsoft\Packages\Products\RM-1075\msimage.mbn -orig_gpt
[17:08:57.045] D_MSG : Process started Tue Mar 01 17:08:57 2016
[17:08:57.056] D_MSG : Debugging enabled for emergency

[17:08:57.058] D_MSG : Initiating emergency download
[17:08:57.078] D_MSG : Using Sahara emergency protocol
[17:08:57.079] D_MSG : QUATTRO EMERGENCY FLASH START
[17:08:57.080] D_MSG : Quattro Emergency Programmer version 2014.11.25.001
[17:08:57.084] D_MSG : ED download selected
[17:08:57.085] D_MSG : Failed to load XML configuration file
[17:08:57.086] D_ERR : Quattro emergency flash exited with error code 85000
[17:08:57.086] D_MSG : QUATTRO EMERGENCY FLASH END
[17:09:02.104] D_MSG : Emergency messaging closed successfully
[17:09:02.107] D_MSG : Operation took about 5.00 seconds.
[17:09:02.109] D_ERR : THOR2 1.8.2.18 exited with error code 85000 (0x14C08)

.hex is renamed (hopefully) MPRG8x26_fh.ede, and .mbn is renamed gpt.bin (tried gpt0.bin too, with the same result)

as i understood from this post, it could be "not corrected", the size does not match, and so on. or it's just different hex for firehose and sahara protocol only.