This is also the format used by the MIME mail encoding standards audio/basic attachments.
Some applications saves these with a 'reverse' bit ordering within each byte. If you have such a file then go to the waveform properties, select 'Mu-law 8-bit reversed' and press apply. To save such a file, simply change the data format and the save dialog box appropriately.
There's no standard file extension for this type of data and there's no way to auto-detect the format, so you'll have to use a .ULAW or .ULW file extension to make the program recognize what it is!