Prepositions related to the Internet and computers
Solution 1:
(I am) in a chat, but on chat
(It is) in a file, but on file
(It is) in a document
(I post / I saw it) on a forum
(I am) on Skype, but in a Skype call
(The file is) on my computer
(It is in) in a folder
The only time something would be in a computer would be if you were talking about the components of the computer like hard disk, memory, wiring, etc.