Access denied error when accessing usb stick files in Windows IoT core app - raspberry-pi2

Question

I want to access files (images, text files etc.) from an USB stick on my Raspberry Pi 2 using Windows 10 IoT Core.
So I've added the to the appxmanifest file.
When using this code in my IBackgroundTask I get an access denied error in the second line:
public sealed class StartupTask : IBackgroundTask
{
public async void Run(IBackgroundTaskInstance taskInstance)
{
//...
Windows.Storage.StorageFolder sf = Windows.Storage.KnownFolders.RemovableDevices;
//get list of drives
IReadOnlyList<Windows.Storage.StorageFolder> list = await sf.GetFoldersAsync();
...
}
}
I found that I should add the fileTypeAssociation with the file types I'd like to access in Package.appxmanifest so I did that:
<?xml version="1.0" encoding="utf-8"?>
<Package xmlns="http://schemas.microsoft.com/appx/manifest/foundation/windows10" xmlns:mp="http://schemas.microsoft.com/appx/2014/phone/manifest" xmlns:uap="http://schemas.microsoft.com/appx/manifest/uap/windows10" xmlns:iot="http://schemas.microsoft.com/appx/manifest/iot/windows10" IgnorableNamespaces="uap mp iot">
<Identity Name="test-uwp" Publisher="CN=user" Version="1.0.0.0" />
<mp:PhoneIdentity PhoneProductId="8f31dff8-3a2b-4df1-90bb-2c5267f32980" PhonePublisherId="00000000-0000-0000-0000-000000000000" />
<Properties>
<DisplayName>test</DisplayName>
<PublisherDisplayName>user</PublisherDisplayName>
<Logo>Assets\StoreLogo.png</Logo>
</Properties>
<Dependencies>
<TargetDeviceFamily Name="Windows.Universal" MinVersion="10.0.0.0" MaxVersionTested="10.0.0.0" />
</Dependencies>
<Resources>
<Resource Language="x-generate" />
</Resources>
<Applications>
<Application Id="App">
<uap:VisualElements DisplayName="test" Square150x150Logo="Assets\Square150x150Logo.png" Square44x44Logo="Assets\Square44x44Logo.png" Description="test" BackgroundColor="transparent" AppListEntry="none">
<uap:DefaultTile Wide310x150Logo="Assets\Wide310x150Logo.png">
</uap:DefaultTile>
<uap:SplashScreen Image="Assets\SplashScreen.png" />
</uap:VisualElements>
<Extensions>
<Extension Category="windows.backgroundTasks" EntryPoint="test.StartupTask">
<BackgroundTasks>
<iot:Task Type="startup" />
</BackgroundTasks>
</Extension>
<uap:Extension Category="windows.fileTypeAssociation">
<uap:FileTypeAssociation Name="myimages">
<uap:SupportedFileTypes>
<uap:FileType ContentType="image/jpeg">.jpg</uap:FileType>
</uap:SupportedFileTypes>
</uap:FileTypeAssociation>
</uap:Extension>
</Extensions>
</Application>
</Applications>
<Capabilities>
<Capability Name="internetClient" />
<uap:Capability Name="removableStorage" />
</Capabilities>
</Package>
If I want to deploy that, I get the following error:
Severity Code Description Project File Line Suppression State
Error Error : DEP0700 : Registration of the app failed.
AppxManifest.xml(37,10): error 0x80070490: Cannot register the
test-uwp_1.0.0.0_arm__yzekw4x8qxe1g package because the following
error was encountered while parsing the windows.fileTypeAssociation
Extension element: Element not found. . Try again and contact the
package publisher if the problem persists. (0x80073cf6)
As soon as I remove the uap:Extension element, the error goes away (but the access denied is still there).
Did I miss anything? Is it not possible to access files from an USB stick using a background service (I want to run that headless with no user interaction)?

Answer 1

At the moment you can't register a headless app that uses filetypeAssociation.
There is a workaround - see here: https://github.com/ms-iot/ntvsiot/issues/62
Simply add a headed app (project) to your solution (no need for any special code there).
Add a reference to this project in your headless app.
Now change the manifest of the headless (file asso..) and add Executable: YourHeadedApp.exe and EntryPoint: YourHeadedApp.App now with the next deploy the EXE will be included in deployment - so it can be found when the manifest is checked.