FreeHand (Adobe FreeHand, Macromedia FreeHand, Aldus FreeHand; it went through several owners) is a now-discontinued vector graphic editor for Windows and Mac. Its native file format is a proprietary one that does not appear to have ever been publicly documented; however, somebody apparently reverse-engineered it to create the libfreehand library to process it. As this library is open-source, examining the sources will probably give some insights into the file structure.
Files are found in a number of extensions (and, for Mac versions, often with no extension at all), but the most common extensions are 'fh' followed by a version number, such as .fh10 for FreeHand 10.
For FreeHand 7, 8, and 9, the first four bytes of the file will spell (in ASCII), AGD2, AGD3, and AGD4 respectively. In FreeHand 10, the text "FreeHand10" is within the first 24 bytes. Some other versions possibly also have "FreeHand" followed by a version number somewhere in the early part of the file.