We were first hosting the wallpaper uploaded by user on the Heroku server which was downloaded by the Travis CI which was previous solution for sending wallpaper to the Travis CI, but the problem was that wallpaper was downloaded only when build started building the ISO.
Downloading the hosted wallpaper was not a problem but the problem in that method was that the wallpaper hosted can be changed if another user also starts the build using Meilix Generator and uploads the wallpaper which will replace the previous wallpaper so simultaneous builds was not possible in previous method and resulted in conflicts.
So we thought of a sending the wallpaper to the Travis CI server for that we used base64 to and encoded it to a string using this.
After uploading we send it as a variable to Travis CI and decode it. We will receive a binary file after decoding now we need to detect the mime type of the file and rename it accordingly before applying for that we use a script like this.
After fixing the wallpaper extension we can apply it using the themes by replacing it with the theme wallpaper.