Download binary/raw file for POST request

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I assume you mean "extract the file from a capture file" instead of "download".

Option 1: In the Wireshark menu go to File / Export Object / HTTP... then select the object you want to export and click 'Save' (or 'Save All').
Also documented at par. 5.7.7. The “Export Objects” Dialog Box.

Option 2: If the binary is one of the items of a multipart POST then select in the 'packet details' pane the data part under "Encapsulated multipart part", right-click and select "Export Packet Bytes..." from the context menu.