Grant read permission to multiple files without starting an intent - android-permissions

Question

I'm coding an App which export all my MMS pics to multiple Download subfolders and I encounter some permission issues.
My code is extracting all my messages (text + images) and creating a html file with them following this tree :
My Download Folder/My App Name/Contact Name/
And in this folder, all the pics + my html file.
So, the paths to the pictures are dynamic and I can't define all of them in my provider_paths.xml.
What I've done :
Basically, at the end of my pic generating process, I need to grant my new generated image the read permission to further open it in a web page of myself.
The difficulty here is to keep the read permission even if the App is destroyed. I need to see my Web page without my App working on it obviously.
I'm using a FileProvider for this and this is what I've tried so far.
I obviously requested READ_EXTERNAL_STORAGE in my Manifest and made it granted by the user.
My Manifest :
<provider
android:name="androidx.core.content.FileProvider"
android:authorities="#string/file_provider_authority"
android:exported="false"
android:readPermission="true"
android:writePermission="true"
android:grantUriPermissions="true">
<meta-data
android:name="android.support.FILE_PROVIDER_PATHS"
android:resource="#xml/provider_paths"/>
</provider>
My provider_paths.xml :
<external-path path="Download/MyApp/" name="Download"/>
The code at the end of the pic generating process :
Uri uriForPic = FileProvider.getUriForFile(MainActivity.this,getString(R.string.file_provider_authority),imagefile);
intent.setDataAndType(uriForPic, "image/jpeg").addFlags(Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_GRANT_READ_URI_PERMISSION);
Not working. So I tried this one :
Uri uriForPic = FileProvider.getUriForFile(MainActivity.this,getString(R.string.file_provider_authority),imagefile);
Intent intent = new Intent();
intent.setAction(Intent.ACTION_VIEW);
Context context = MainActivity.this;
List<ResolveInfo> resInfoList = context.getPackageManager().queryIntentActivities(intent, PackageManager.MATCH_DEFAULT_ONLY);
for (ResolveInfo resolveInfo : resInfoList) {
String packageName = resolveInfo.activityInfo.packageName;
context.grantUriPermission(packageName, uriForPic, Intent.FLAG_GRANT_WRITE_URI_PERMISSION | Intent.FLAG_GRANT_READ_URI_PERMISSION);
}
All is doing smooth until I try to open my html pages with my pics in them. Then, I got the java.lang.SecurityException: Permission Denial error. This is weird since I can watch all my pics using the gallery without any exception.
I understand I must manually grant permission to each of my FileProvider generated Uris but these solutions don't work and after a week trying and googling, I'm lost and need your help...
So, two questions. Does my provider_paths.xml grant access to subfolders ? And what is missing in my understanding of permissions ?
Thanks for reading me,