Update BulkUploadDescPrompt string to take a string instead of uint for bytes

With this change, it's important you don't try to use a newer text.dat with an
older sbbs (and perform a ;UPLOAD sysop operation).

Previously, each file's size in (k)ibibytes was displayed, which is not very
modern of us. Use the byte estimate (e.g. "1.2M") strings intead.