How to backup your official ROM to flash it anytime with Flashtool
If you do not want to lose the warranty of your Xperia phone with an locked bootloader, you have to backup your official ROM.
download Sony Update Service (SUS) and reinstall firmware of your phone (backup your data first if you want to). This process erase also the ROOT (if you had rooted your phone). SUS cannot reinstall the firmware if you have an unlocked bootloader.
after that, go to SUS folder in Program Files/Program Files (x84)>Sony Mobile>Update Service>db>"some random number">blob_fs. There you will find 3 or 4 files named "FILE_xyz" (where xyz are some random numbers). Copy them and keep the original.
create a new folder somewhere else (on Desktop for example) and paste those 3-4 files from blob_fs into the new folder you created
download and install latest Flashtool (at the moment of the tutorial, I used the version 0.9.16.1)
open the Flashtool, click on Tools>Bundles>FILESET Decrypt, choose the new folder where you saved those 3-4 FILE files, then select and move them all from the left to the right column.
click Convert and after approx. one minute a pop up window will appear via Flashtool
select all the new files from left and transfer them to the right column
double click on Device: empty box and choose the phone model you have with double click (C210X, Sony Xperia L > C2105 or C2104). My phone is a C2105. If you phone is C2104, select C2104.
at Branding: write what you want. For example Telcel Mexico. If it's unbranded write Unbranded Mexico.
at Version: write the numbers you find from your phone at Settings>About phone>Build number. For example 15.0.A.2.17 it's mine.
now click Create, select again the phone model and Flashtool will create backup of your original firmware customization, that you can anytime flash to your phone. The ftf file will be saved in the folder where you pasted FILE files. The process takes a while, 30-60 minutes (it depends on your PC computing power).
the output folder will be the C:>Flashtool>firmwares where you pasted those 3-4 FILE files and it will have this name: PhoneModel_BuildNumber_Branding.ftf (e.g. C2105_15.0.A.2.17_Telcel_Mexico.ftf)
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