I have my own collection, but these Internet resources cover everything I had PLUS. Good resources for files looking up file extensions:

• Almost Every File Format in the World
• Data Formats
• Every File Format in the World
• Compressed Image File Formats : JPEG, PNG, GIF, XBM, BMP
• Graphics File Formats FAQ
• JPEG FAQ
• Microsoft RIFF
• Microsoft Windows Clipboard
• Microsoft Windows Cursor and Icon
• Microsoft Windows Metafile
• Multimedia File Formats on the Internet
• Wotsit's Format