Files associated with messages are stored in the folder specified in the second Home card (normally "Uploads"). The file name is composed of the name of the message section plus the date/time of the message converted into seconds. This isn't the most Sysop-friendly naming convention in the world, but it ensures that the connection between the name and the message is maintained even if the message is moved to a different card.
Mac files are generally converted to MacBinary form in the process of uploading. They can be converted to and from MacBinary locally by pressing the upload or download button when in local operation mode.