I should split something into a bunch of 10MB .rar archives, rename them all to .jpg and post it on here, as a lark
alternatively, use .zip - and concat the .zip data onto the end of an actual .jpg
.jpg data is read from the start of the file, .zip data is read from the end of the file,1 so you can concat them together and make a polyglot file that's valid as both
Note: it's been a while since i did this and my memory is probably not 100% accurate
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for complicated reasons that boil down to "it's the easiest way for the format to support append-only edits"
