Both directives seem to define how the contents of a file with a given extension should be threatened but when do you need the first one and when the second?
Strict versus quirks mode
Something slightly related is how browsers deal with the content type you set in the AddType directive. Gecko-based browsers (like Firefox) follow the standard closely so they will handle a file according to the content type from the Apache configuration. An image would show up as text if you wrongly assigned the text/plain content type. Internet Explorer is more flexible and tries to guess the mime type for misconfigured web servers showing a wrongly defined image as, well, an image.