The problem is caused by the Mac user typing the filename including what he thinks the right suffix is. Macs don't need a file suffix to work out what file type they are dealing with. Instead they have another 'invisible' file called a resource fork which includes the file type information.

The File Save format is somewhere obvious in the save as dialog but the user has to actually look at it. It is much the same as when saving a Word file in Windows so the different OS is not a hindrance to your being able to show him where he is going wrong.

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Andrew Lockton
Chrysalis Design, Melbourne Australia