If you are sure you’re only ever working with images, you can check out the exif_imagetype() PHP function, which attempts to return the image MIME type.
If you don’t mind external dependencies, you can also check out the excellent getID3 library which can determine the MIME type of many different file types.
Lastly, you can check out the mime_content_type() function – but it has been deprecated for the Fileinfo PECL extension.