Download and make your own Firmware
Don't misread the title. This tutorial will help you download the needed FILE_xxxxxxxx so that FlashTool can decrypt them and create a bundle to flash.
Requirements:
- Download the XperiaFirmware Downloader tool thing here: http://forum.xda-developers.com/cro...xperifirm-xperia-firmware-downloader-t2834142
- Flashtool (use search for this, there are a lot of threads for it)
- Patience
- A brain
Steps:
First Download the needed programs and install Flashtool and its drivers. For extra explanation you can check my how to use flashtool tutorial. Download the XperiaFirmware Download tool (see link above) and unzip it.
Open it:

Select your deviceand the wanted customatisation number or operator and select the wanted firmware version:

A new window will pop up and select download and choose an location:

Let it download:

Close the window and the program...
Open flashtool and select Tools > Bundle > FILESET Decrypt:

Select the folder with the FILE's and add them to the list on the right:

Flashtool will decrypt it:

A new window pops up and you can create your bundle; select your device, enter the branding and the firmware version and select create:

Let flashtool do it's job and after a while it is done:

The .ftf file is in the same place as the FILE files.. So look up there and share the .ftf in the correct forum!
Good Luck!