In the quick reply form, a magnet icon will appear.
You need to select a png file to upload first, as you would normally (this is what the file will appear as), then click on the magnet to select a file to embed.
This works by appending the file in the last IDAT chunk.
Metadata information is stored in a tEXt chunk, placed near the header so that a parser looking for that embedded information can bail out without having to parse the whole file.
CUM[null]0 -> The last IDAT chunk is formatted as [filename length[LE 4 bytes], filename, filedata]
CUM[null]X is reserved for future extensions
Possible workaround for 4chan jannies would be to assoome IDAT chunks don't go over a certain size, slightly harder workaround would be to check if the deflate stream yields enough pixels to fit the described dimensions of the image.
Other formats
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The format used by Zip anon won't be supported because:
- it isn't extensible (if change were to the storage format, an extension update wouldn't be backcompatible)
- requires parsing the whole file to know if it has an embedded file
- and includes a private chunk type that is functionally defective (stores the length of the last IDAT chunk as a mean to identify it, instead of assooming it's simply the last one)
