If you run a music sharing website and allow uploads and downloads of audio files and someone tricks your application into accepting images by making them appear as if they were audio files—that seems very different to me from say a site that just allows any file under a certain limit to be uploaded and stored.

In both cases the images are received by the server and stored in the database (so you’re correct to say there is no difference in that sense) but the principle is very different. In the former case it is clear that the user is violating the website’s terms of service and engaging in deception to accomplish the goal with zero support from the webserver. In the latter case the website is signaling that this uploading images is an acceptable use.