add API method to get File based on name, and fix permission validation for Folder

Oqtane.Client/Services/Interfaces/IFileService.cs

@@ -1,5 +1,6 @@
 using Oqtane.Models;
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 using System.Threading.Tasks;
 
 namespace Oqtane.Services
@@ -33,6 +34,15 @@ namespace Oqtane.Services
         /// <returns></returns>
         Task<File> GetFileAsync(int fileId);
 
+        /// <summary>
+        /// Get a <see cref="File"/> based on the <see cref="Folder"/> and file name.
+        /// </summary>
+        /// <param name="folderId">Reference to the <see cref="Folder"/></param>
+        /// <param name="name">name of the file
+        /// </param>
+        /// <returns></returns>
+        Task<File> GetFileAsync(int folderId, string name);
+
         /// <summary>
         /// Add / store a <see cref="File"/> record.
         /// This does not contain the file contents.