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.

References

http://bitsavers.org/pdf/apple/mac/a_ux/aux_2.0/030-0787-A_AUX_Toolbox_Macintosh_ROM_Interface_1990.pdf from "A/UX Toolbox: Macintosh ROM Interface manual", Chapter 6 File Systems and File Formats

identifies the magic number 0x00051607 for AppleDouble files as the first 4 bytes in the file

https://en.wikipedia.org/wiki/AppleSingle_and_AppleDouble_formats background and additional information

https://www.ietf.org/rfc/rfc1740.txt MIME Encapsulation of Macintosh files - MacMIME