generateDosExternalFileAttr = function (dosPermissions, isDir) {

    // the dir flag is already set for compatibility
    return (dosPermissions || 0)  & 0x3F;
}
Generate the DOS part of the external file attributes.
@param {Object} dosPermissions the dos permissions or null.
@param {Boolean} isDir true if the entry is a directory, false otherwise.
@return {Number} a 32 bit integer.

Bit 0     Read-Only
Bit 1     Hidden
Bit 2     System
Bit 3     Volume Label
Bit 4     Directory
Bit 5     Archive