Type 3 (cover) is essentially a GIF image file. You can just copy any GIF image's binary data you want, as long as it's less than 65536 bytes. (Nesbox was wrong, only 2 bytes for size are actually being read, the 3rd byte is unused.)
Type 5 (code) is plain text. All other valid types are based on the TIC-80 RAM layouts for each of those assets.
Useful wiki pages:
https://github.com/nesbox/TIC-80/wiki/tic-File-Format
https://github.com/nesbox/TIC-80/wiki/RAM