[Xperia Z4 Tablet Q&A, Help & Troubleshooting] jelbo:
I'm pretty sure it's not a file on a filesystem and, if it is, it would be on a partition you can not just search on easily. I remember having to flash a boot splash image on my HTC One m7 using fastboot flash splash1 splash.img. splash1 is the dedicated partition for the splash image, on mmcblk0p13.

I don't know how it is on Xperia's, but I guess it's similar. It'd be nice to be able to change this though, I also dislike the bright white Sony splash screen.

Here's the partition info I pulled from Disk Info Pro:

--------------------------
Internal Storage (MMC)
--------------------------
* TA [mmcblk0p1] Not mounted
Total space: 2 MB
* LTALabel [mmcblk0p2] (/lta-label) [ext4]
Used: 5.7 MB, Free: 10.3 MB, Total space: 16 MB
* modem [mmcblk0p3] (/firmware) [vfat]
Used: 16.8 MB, Free: 63.2 MB, Total space: 80 MB
* pmic [mmcblk0p4] Not mounted
Total space: 128 KB
* alt_pmic [mmcblk0p5] Not mounted
Total space: 128 KB
* limits [mmcblk0p6] Not mounted
Total space: 1 KB
* DDR [mmcblk0p7] Not mounted
Total space: 1 MB
* apdp [mmcblk0p8] Not mounted
Total space: 256 KB
* msadp [mmcblk0p9] Not mounted
Total space: 256 KB
* dpo [mmcblk0p10] Not mounted
Total space: 1 KB
* hyp [mmcblk0p11] Not mounted
Total space: 512 KB
* alt_hyp [mmcblk0p12] Not mounted
Total space: 512 KB
* fsg [mmcblk0p13] Not mounted
Total space: 1.5 MB
* ssd [mmcblk0p14] Not mounted
Total space: 8 KB
* sbl1 [mmcblk0p15] Not mounted
Total space: 1 MB
* alt_sbl1 [mmcblk0p16] Not mounted
Total space: 1 MB
* modemst1 [mmcblk0p17] Not mounted
Total space: 1.5 MB
* modemst2 [mmcblk0p18] Not mounted
Total space: 1.5 MB
* s1sbl [mmcblk0p19] Not mounted
Total space: 256 KB
* alt_s1sbl [mmcblk0p20] Not mounted
Total space: 256 KB
* sdi [mmcblk0p21] Not mounted
Total space: 128 KB
* alt_sdi [mmcblk0p22] Not mounted
Total space: 128 KB
* tz [mmcblk0p23] Not mounted
Total space: 1 MB
* alt_tz [mmcblk0p24] Not mounted
Total space: 1 MB
* rpm [mmcblk0p25] Not mounted
Total space: 512 KB
* alt_rpm [mmcblk0p26] Not mounted
Total space: 512 KB
* aboot [mmcblk0p27] Not mounted
Total space: 1 MB
* alt_aboot [mmcblk0p28] Not mounted
Total space: 1 MB
* boot [mmcblk0p29] Not mounted
Total space: 64 MB
* rdimage [mmcblk0p30] Not mounted
Total space: 32 MB
* persist [mmcblk0p31] (/persist) [ext4]
Used: 5.3 MB, Free: 26.7 MB, Total space: 32 MB
* FOTAKernel [mmcblk0p32] Not mounted
Total space: 64 MB
* misc [mmcblk0p33] Not mounted
Total space: 1 MB
* keystore [mmcblk0p34] Not mounted
Total space: 512 KB
* devinfo [mmcblk0p35] Not mounted
Total space: 1 KB
* config [mmcblk0p36] Not mounted
Total space: 512 KB
* rddata [mmcblk0p37] Not mounted
Total space: 100 MB
* apps_log [mmcblk0p38] (/rca) [ext4]
Used: 5.2 MB, Free: 4.8 MB, Total space: 10 MB
* diag [mmcblk0p39] (/idd) [ext4]
Used: 2.5 MB, Free: 13.5 MB, Total space: 16 MB
* oem [mmcblk0p40] (/oem) [ext4]
Used: 79.2 MB, Free: 220 MB, Total space: 300 MB
* Cache [mmcblk0p41] (/cache) [ext4]
Used: 11.4 MB, Free: 188 MB, Total space: 200 MB
* B2B [mmcblk0p42] Not mounted
Total space: 384 MB
* Data (userdata) [mmcblk0p43] (/data) [ext4]
Used: 13.7 GB, Free: 9 GB, Total space: 22.7 GB
* System [mmcblk0p44] (/system) [ext4]
Used: 4.3 GB, Free: 820 MB, Total space: 5.1 GB
* mmcblk0rpmb [mmcblk0rpmb] Not mounted
Total space: 4 MB
--------------------------
tmpfs mount points
--------------------------
* /dev [tmpfs]
Used: 144 KB, Free: 1.4 GB, Total space: 1.4 GB
* /mnt [tmpfs]
Used: 0 B, Free: 1.4 GB, Total space: 1.4 GB
* /tmp [tmpfs]
Used: 0 B, Free: 1.4 GB, Total space: 1.4 GB
* /storage [tmpfs]
Used: 0 B, Free: 1.4 GB, Total space: 1.4 GB
* /storage/self [tmpfs]
Used: 0 B, Free: 1.4 GB, Total space: 1.4 GB