Any decent application that open a file will check if it is in correct format by checking some predefined markers in them. For example every bmp file starts with BM, pngs start with PNG, JPEG and variant contain some text like JFIF inside of them, the they continue with specific information like format version etc. this kind of information is not limited to image files btw, exe files start with MZ for example, zip files with PK, rar files with RAR