If you must know (but I recommend against writing code to manipulate item
files when you could use the API.pm interface),
the file format is:

A series of name-value pairs that look like mail headers define the item
properties:
        Parent: 1
        LastModified: 1997-Aug-13 12:40pm

This is followed by a series of part definitions. Each one starts with a
"Part: n" header, with n counting from 0 up. After the Part: header comes
the properties for the part (more name-value pairs). The last property
is a Lines: field that tells how many lines of text make up the part's
Text property; this field is followed by exactly that many lines in the
file, then an "EndPart: n" field. After that is another Part: or the end of
file.