Appledouble header file
A separate file in a filesystem containing the Apple resource fork for a related data file, developed by Apple for A/UX and therefore compatible with other filesystems including those mounted remotely. They are heavily used under Mac OS X.
This file is named the same as the data file, with "._" prepended to the original name. These header files are automatically included in ZIP files under a hierarchy starting with "__MACOSX" when utilizing the Finder's "Archive as..." function.
Per "A/UX Toolbox: Macintosh ROM Interface manual", Chapter 6 File Systems and File Formats, AppleDouble files can be idenfied by the magic number 0x00051607 stored in the first 4 bytes of the file.