But the website
did specify a format. Like I said in my sibling comment, a lack of an `accept` attribute (which is the same as saying `accept="⋆/⋆"`) has a conventional meaning from a plethora of legacy use-cases; and that meaning is:
"Give me the underlying data, just as it is. You may or may not understand what it is, but I'm asking you to pretend that you don't, because I definitely don't understand what it is. I'm acting as a courier on behalf of a recipient who wants whatever you give me. All they told me was to get it from you. Please don't try to guess why they want it, or to prepare it for them in some way. Their motivations are beyond our understanding. They just want it. They want what you have, the way you currently have it. Do as little to it as possible, because anything you do may be counter-productive to their unknowable designs."
This is, for one thing, the use-case of file-sharing websites. If you upload something to e.g. MEGA, or WeTransfer, you're doing that in order for something further to happen to it on the other side. The other side may or may not have wanted the file in its original format, but that question is up to them, not up to the sender. The job of a "dumb pipe" file-transfer service, is to take what it's given, and losslessly pass it on to the recipient, so that further steps can happen. And, as such, it's also a responsibility of a file-transfer service to ask the User-Agent to also send the file on to it losslessly, because in this case the User-Agent is also acting as part of the "dumb pipe" transfer process.
Let me put it this way: if my photos were not saving correctly, and someone at Apple asked me to file a Radar ticket and attach such a mis-encoded photo to the ticket... how would the Radar web-app express the intent of "I want the stupid mis-encoded file that you-the-device are using to store what you think is a Photos photo"? Well, our legacy convention is that it would express that intent through `accept="⋆/⋆"`. (Or a lack of an `accept` header at all.)
Note that this is different from an `accept` attribute like "image/⋆". In that case, we know something—we know that the recipient we're acting as courier has the intent to use the uploaded file as an image—so both the mis-encoded file, and maybe HEIC files, are probably bad candidates. One should be filtered out as an upload candidate; the other should maybe be transcoded (just like a RAW camera file would be.)