Backport #17406.
Closes#17378
Both errors from #17378 were caused by #15175.
Problem 1 (error with added file):
`ToUTF8WithFallbackReader` creates a `MultiReader` from a `byte[2048]` and the remaining reader. `CreateReaderAndGuessDelimiter` tries to read 10000 bytes from this reader but only gets 2048 because that's the first reader in the `MultiReader`. Then the `if size < 1e4` thinks the input is at EOF and just returns that.
Problem 2 (error with changed file):
The blob reader gets defer closed. That was fine because the old version reads the whole file into memory. Now with the streaming version the close needs to defer after the method.
Co-authored-by: zeripath <art27@cantab.net>
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