How to associate iPhone application with _every_ file type?

Question

There is iPhone app "Another Mail Client" that should be able to open any file to send it as attachment. So, I want to associate this application with any file with any extension.

Following the documentation, we should declare support for files with the root UTI-type public.data – any file should belong to this type. It works, but not at all. In this case, our app will not be able to open any file, but only those which have already been registered in the system. For example, if in any application (e.g., dropbox) we'll try to "open in..." file with an unknown extension (file.unknowntype) using UIDocumentInteractionController, then the answer will be negative despite the fact that we have already registered our application and it supports the root UTI-type public.data. But, if you install another application, which supports files with extension (*.unknowntype), then our application will also be able to open these files and will appear in "open in..." application list.

UPD: @Gabriel This is CFBundleDocumentTypes part of my info.plist file:

<key>CFBundleDocumentTypes</key>
<array>
    <dict>
        <key>CFBundleTypeName</key>
        <string>MyMail</string>
        <key>LSItemContentTypes</key>
        <array>
            <string>public.data</string>
        </array>
        <key>CFBundleTypeRole</key>
        <string>Viewer</string>
        <key>LSHandlerRank</key>
        <string>Default</string>
        <key>CFBundleTypeIconFiles</key>
        <array>
            <string>Icon29.png</string>
            ...
            <string>Icon114.png</string>
        </array>
    </dict>
</array>

Answer 1

I've made an app with the following setup

<key>CFBundleDocumentTypes</key>
<array>
    <dict>
        <key>CFBundleTypeIconFiles</key>
        <array/>
        <key>CFBundleTypeName</key>
        <string>name</string>
        <key>LSItemContentTypes</key>
        <array>
            <string>public.data</string>
        </array>
    </dict>
</array>

When I try to open a .pdf from Safari, this app shows up in "open in.." list. Can you make a sample app and try it?

UPD:

It seems like claim 'public.data' (tried also public.item, public.content) means file, which belongs to set "all known to system UTIs", not any file. So, you will be able to handle 99% of files, which users want to send by email , but not all. Another way would be to export UTI that you think are important, but which are not in system UTIs by default.