(or '' if no SubfileType tag).
    Valid only in EXIF directories for tags after 0x00fe.</td></tr>
    
<tr><td>TAG_EXTRA</td><td>Hash of extra tag information keyed by tag key.  Current extra
information may include:
    <table class='clear'>
    <tr valign='top'><td>G0</td><td>-</td><td>override for family 0 group name</td></tr>
    <tr valign='top'><td>G1</td><td>-</td><td>dynamic family 1 group name (eg. IFD name or XMP
        namespace), with a leading '+' to add value to the existing group 1 name</td