A file is simply a chunk of data that is persisted, usually, on some hard medium such as a hard drive. Files are normally composed of a sequence of bytes whose encoding maps onto some other pattern, like a sequence of numbers or electrical pulses. A nearly infinite number of encodings are possible, with some common ones being text files, image files, and music files. Files have a fixed length, and to be read, their character encoding must be deciphered by some sort of reader, such as an MP3 player or a word processor.
When a file is in transit, moving through a cable after it's been siphoned off of some storage device, it is no different than any other data stream running through the wire. Its previous solid state is just a stable blueprint that can...