if you open the files in notepad or change the jpg to .txt and a exe to .txt you will see the first X amount of bytes defines what type of file it is etc. I have never looked into where the "standard" is but as you will see all JPEGS start with a specific byte and EXE start with a specific byte no matter what the content
Also JPEG is a licensed compressed form of an image and BMP is Microsoft Windows version of an image(with little or no compression I believe. png is open source or GPL licensed and technically your files do not need to be "licensed" to convert to JPEG. This is the same as a .MP3 vs a.OGG in terms of music