Storage sharing with standard apps concept

In short:
My app should let the user take and display photos using the standard apps.
The photos should not show up in the Gallery.
The photos should be removed when the app is de-installed.

What I got so far:
Since I want to use the device's default camera app, I need to use a shareable place for the files.
I tried using 'ExternalFilesDir' with a 'Manifest/xml/provider' contructed Uri in the calling
intent. This seams to work with the camera app:

private String mImagePath;

private void takePhoto(){
    Intent intent = new Intent(MediaStore.ACTION_IMAGE_CAPTURE);
    if (intent.resolveActivity(getPackageManager()) == null) return;

    String filename="Image" + System.currentTimeMillis();
    File dir = context.getExternalFilesDir(Environment.DIRECTORY_PICTURES);
    File file =null;
    try{
        file = File.createTempFile(filename,".jpg", dir);
    }catch(Exception ex){
        ex.printStackTrace();
    }
    if (file == null) return;

    mImagePath =file.getAbsolutePath();
    Uri uri = FileProvider.getUriForFile(this,getApplicationContext().getPackageName()+".provider", file);
    intent.putExtra(MediaStore.EXTRA_OUTPUT, uri);
    startActivityForResult(intent, requestcode);
}

This part seams to work (due to my 'competence' I cannot check the folder since the Android Studio's Device File Eplorer does not let me, grrh!).

But when I try to display the images, all the standard apps (Gallery, Foto etc.) throw an 'access denied' error message.

private void displayPhoto{
        Intent intent = new Intent();
        intent.setAction(Intent.ACTION_VIEW);
        Uri contentUri = FileProvider.getUriForFile(this,getApplicationContext().getPackageName()+".provider", new File(mImagePath));
        intent.setDataAndType(contentUri, "image/*");
        startActivity(intent);
}

My questions are:

• Why can the camera app store a file in ExternalFilesDir but the
  displaying app cannot read it from there?

-or-

• Is there something wrong with my code?
• Can my app somehow grant access to the displaying app?
• What other concept could I use to achieve my targets?
• How can I browse the content of ExternalFilesDir within Android
  Studio or another tool?

I do not want to write my own displaying code since the user is familliar with the standard app.
I need to use minSDKVersion 22.

Thanks for helping me here!

Meanwhile, after hours and hours, I found the reason why Gallery or other apps show the 'access denied' error:
Im addition to the uri the caller app has to set the 'FLAG_GRANT_READ_URI_PERMISSION' flag in the calling intent. Like this:

Intent intent = new Intent();
intent.setAction(Intent.ACTION_VIEW);
intent.setDataAndType(mycontenturi, "image/*");
intent.setFlags(FLAG_GRANT_READ_URI_PERMISSION);
startActivity(intent);

Also I found that the only entries in the fileprovider xml which work are:

<?xml version="1.0" encoding="utf-8"?>
<paths xmlns:android="http://schemas.android.com/apk/res/android">
   <external-files-path name="my_images" path="Pictures/" />
   <external-files-path name="my_images" path="." />
   <external-files-path name="camera_image" path="."/>
   <external-files-path name="camera_image" path="Pictures/"/>
</paths>

Funny enough: when path is "." it creates a Pictures subdir and when it says "Pictures/" it doesn't.