The `file-size-human-readable' function is very useful but could do with some better formatting: normally, a space goes between the number and unit; you don't write '3kg' or '25m/s' but '3 kg' and '25 m/s' (sloppy British newspapers notwithstanding). We could add an optional argument so that the caller can use the spacing of preference; the default should probably be no space, for compatibility.

For some reason, only the `iec' mode adds an actual unit (B) to the result; the default and `si' modes just append a scale prefix. Of course a user can append the unit of choice, as in:

  (concat (file-size-human-readable size 'si) "B")

which permits the function to be used for other units than bytes, such as "bit/s" (although the name makes it clear that it is intended for file sizes only). However, spacing complicates things, since we want

  (file-size-human-readable 14 'si " ")

to return "14", not "14 ", but the latter is what we need when appending the unit.
I'm not sure how to fix this. We could add another optional argument, UNIT say, which is the string to use as unit, defaulting to "B" in `iec' mode and the empty string otherwise. The attached patch does not address this.

There is also a small glitch to be fixed:

 (file-size-human-readable 3 'iec) => "3iB"

which of course should be "3B".