My document icon does not show in Mac Mail since Sierra

Hi,

I assigned three file extensions and a MIME type to my application. Upon Mac OSX 10.11, it worked for both Finder and Mac Mail. Now, since Sierra (10.12), the Mac Mail does no longer show the document icon. Seems like Finder does after a while but Mac Mail always using the default icon. I have some customers start complaining about and I can reproduce here.

Sadly, for me the Info.plist seems fine and the two assigned icons (icns) are inside the .app below /Contents/Resources/.

This is the used Info.plist located directly in /Contents/ folder:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
    <key>CFBundleIdentifier</key>
    <string>com.regify.client.mac</string>
    <key>CFBundleName</key>
    <string>regify client</string>
    <key>CFBundleDevelopmentRegion</key>
    <string>English</string>
    <key>CFBundleExecutable</key>
    <string>regify client</string>
    <key>CFBundleInfoDictionaryVersion</key>
    <string>6.0</string>
    <key>CFBundlePackageType</key>
    <string>APPL</string>
    <key>CFBundleDisplayName</key>
    <string>regify client</string>
    <key>CFBundleVersion</key>
    <string>0.1</string>
    <key>CSResourcesFileMapped</key>
    <true/>
    <key>CFBundleIconFile</key>
    <string>regify_128x128.icns</string>
    <key>CFBundleLocalizations</key>
    <array>
        <string>de</string>
        <string>en</string>
        <string>fr</string>
    </array>
    <key>CFBundleDocumentTypes</key>
    <array>
        <dict>
            <key>CFBundleTypeMIMETypes</key>
            <array>
                <string>application/vnd.regify</string>
            </array>
            <key>CFBundleTypeName</key>
            <string>regify message</string>
            <key>LSHandlerRank</key>
            <string>Default</string>
            <key>CFBundleTypeRole</key>
            <string>Editor</string>
            <key>CFBundleTypeIconFile</key>
            <string>regify_doc.icns</string>
            <key>CFBundleTypeExtensions</key>
            <array>
                <string>rgf</string>
                <string>rif</string>
                <string>rgk</string>
            </array>
            <key>CFBundleTypeIconFiles</key>
            <array>
                <string>regify_doc.icns</string>
            </array>
        </dict>
    </array>
</dict>
</plist>

Any idea why it stops working since Sierra and High Sierra? On Darwin and Mavericks I was able to test and here it works fine with the same .app and the icons show in Mac Mail and Finder.