What is the purpose of the file Fiona..meaning if he is interested in using your photos why would he want to send you something in an unusual file format?
mime types usually tell your browser what to do with the file, they are usually types of audio, video or flash..each has its own mime type and by changing that you can effect what programs will be used to open it (flash, micromedia shockwave, windows media player etc) and how the file will be used.
Its not a simple subject to grasp easily but wiki has one of the best explinations on the subject.
MIME - Wikipedia, the free encyclopedia
My advice would be not to open it and for the potential customer to send you the file or a link to the information in a format that is easily viewable for you.
Hope that helps Fi
