It doesn't have to look "exotic" at all: Sam just sends Joe a file in base 64. But that is how all the multimedia content is sent in e-mail now. Until dig into what Sam sent, it looks like just another JPG file sent in e-mail. But translate it from base 64 and give it to an image program and will discover that the file is not a JPG file. Or a PNG, BMP, MP3, WAV, EXE etc. file either. But about have to translate from base 64 to conclude this.