Attachment upload speed units are wrong

Such speed units like Mb/s are given when uploading attachments. Small "b" would mean bits, not bytes, however I think the given number is really bytes. The correct unit would be MiB/s (in English), Mio/s (in French), which would make it overly clear what the unit means. At least the b should be upper cased.

See https://en.wikipedia.org/wiki/Mebibyte#Multiple-byte_units