• DWG - starts with AC
• EXE
  • 16 bit - starts with MZ
• BMP - starts with BM
• GIF - starts with GIF89
• JPEG - 6 bytes then JFIF, 6 bytes might always be FF D8 FF E0 00 10
• PDF - starts with %PDF-
• PNG - starts with 0x89 PNG 0x0D 0x0A 0x1A 0x0A
• XLS - 09 08 10 00 00 06 05 00 at offset 512
• OFX, QXF

References:

• BMP file format http://en.wikipedia.org/wiki/BMP_file_format
• Graphics Interchange Format http://en.wikipedia.org/wiki/Graphics_Interchange_Format
• JPEG File Interchange Format http://en.wikipedia.org/wiki/JPEG_File_Interchange_Format
• Portable Document Format http://en.wikipedia.org/wiki/Pdf
• Portable Network Graphics http://en.wikipedia.org/wiki/Portable_Network_Graphics
• http://www.garykessler.net/library/file_sigs.html
• Motorola S-Record