Custom UTI does not work for iMessage in iOS 10

Question

I defined and used a custom UTI type for my iOS app for iOS 9. Now, after using XCode 8 and iOS 10, I cannot "open" the file, if it is sent via iMessage. If I tap on the icon, nothing happens, although the correct icon is displayed. If it is sent via E-Mail, I can still import it to my app. Is there a difference for mail and iMessage?

My info.plist declarations look like this:

    <key>CFBundleDocumentTypes</key>
<array>
    <dict>
        <key>CFBundleTypeIconFiles</key>
        <array>
            <string>Icon-60@3x</string>
        </array>
        <key>CFBundleTypeName</key>
        <string>My Voting Lists File</string>
        <key>CFBundleTypeRole</key>
        <string>Editor</string>
        <key>LSHandlerRank</key>
        <string>Owner</string>
        <key>LSItemContentTypes</key>
        <array>
            <string>com.my-org.document.lists</string>
        </array>
    </dict>
</array>

and

<key>UTExportedTypeDeclarations</key>
<array>
    <dict>
        <key>UTTypeConformsTo</key>
        <array>
            <string>public.text</string>
            <string>public.data</string>
        </array>
        <key>UTTypeDescription</key>
        <string>My Voting List File</string>
        <key>UTTypeIdentifier</key>
        <string>com.my-org.document.lists</string>
        <key>UTTypeTagSpecification</key>
        <dict>
            <key>public.filename-extension</key>
            <array>
                <string>mylist</string>
            </array>
        </dict>
    </dict>
</array>

I also searched for a solution, but did not find something that works for me. This stackoverflow discussion did not help me :( Any ideas what changed or what my mistake is? Thanks in advance!