Added PowerPoint templates to the Slide Assistant (#1055)

Co-authored-by: Thorsten Sommer <SommerEngineering@users.noreply.github.com>

app/MindWork AI Studio/Tools/DroppedFile.cs

@@ -0,0 +1,42 @@
+using AIStudio.Tools.Rust;
+
+namespace AIStudio.Tools;
+
+/// <summary>
+/// Picks the file which a picker for exactly one file takes from the paths dropped onto it.
+/// </summary>
+public static class DroppedFile
+{
+    private static string TB(string fallbackEN) => PluginSystem.I18N.I.T(fallbackEN, typeof(DroppedFile).Namespace, nameof(DroppedFile));
+
+    /// <summary>
+    /// Finds the first dropped path which leads to a file of an allowed type.
+    /// </summary>
+    /// <remarks>
+    /// A picker carries exactly one file, so a multi-selection cannot be honored as a whole. A dropped
+    /// folder is rejected instead of being read as "the first file inside it": the user was asked for a
+    /// file, and picking one for them would be a surprise.
+    /// </remarks>
+    /// <param name="paths">The dropped paths, in the order the runtime delivered them.</param>
+    /// <param name="filter">The allowed file types, or none to allow every file.</param>
+    /// <returns>The path of the file to take, or null when no dropped path leads to a usable file.</returns>
+    public static string? FirstUsable(IReadOnlyList<string> paths, FileTypeFilter[]? filter) => paths.FirstOrDefault(path => File.Exists(path) && (filter is not { Length: > 0 } || FileTypes.IsAllowedPath(path, filter)));
+
+    /// <summary>
+    /// Explains why none of the dropped paths could be used.
+    /// </summary>
+    /// <param name="paths">The dropped paths.</param>
+    public static string WarningFor(IReadOnlyList<string> paths)
+    {
+        // Naming the actual mistake beats a generic "that did not work". Dropping a folder onto a
+        // file picker is the likeliest of them:
+        if (paths.Any(Directory.Exists))
+            return TB("Please drop a file, not a folder.");
+
+        // The file exists, so the file type filter is what turned it down:
+        if (paths.Any(File.Exists))
+            return TB("Please drop a file with a supported file type.");
+
+        return TB("The dropped file could not be accessed. Please choose it with the file chooser instead.");
+    }
+}