Update attachment checks for embed urls

src/Discord.Net.Core/Entities/Messages/EmbedBuilder.cs

@@ -408,22 +408,22 @@ namespace Discord
             if (Length > MaxEmbedLength)
                 throw new InvalidOperationException($"Total embed length must be less than or equal to {MaxEmbedLength}.");
             if (!string.IsNullOrEmpty(Url))
-                UrlValidation.Validate(Url);
+                UrlValidation.Validate(Url, true);
             if (!string.IsNullOrEmpty(ThumbnailUrl))
-                UrlValidation.Validate(ThumbnailUrl);
-            if (!string.IsNullOrEmpty(ImageUrl) && !ImageUrl.StartsWith("attachment://", StringComparison.Ordinal))
-                UrlValidation.Validate(ImageUrl);
+                UrlValidation.Validate(ThumbnailUrl, true);
+            if (!string.IsNullOrEmpty(ImageUrl))
+                UrlValidation.Validate(ImageUrl, true);
             if (Author != null)
             {
                 if (!string.IsNullOrEmpty(Author.Url))
-                    UrlValidation.Validate(Author.Url);
+                    UrlValidation.Validate(Author.Url, true);
                 if (!string.IsNullOrEmpty(Author.IconUrl))
-                    UrlValidation.Validate(Author.IconUrl);
+                    UrlValidation.Validate(Author.IconUrl, true);
             }
             if(Footer != null)
             {
                 if (!string.IsNullOrEmpty(Footer.IconUrl))
-                    UrlValidation.Validate(Footer.IconUrl);
+                    UrlValidation.Validate(Footer.IconUrl, true);
             }
             var fields = ImmutableArray.CreateBuilder<EmbedField>(Fields.Count);
             for (int i = 0; i < Fields.Count; i++)

src/Discord.Net.Core/Utils/UrlValidation.cs

@@ -9,14 +9,15 @@ namespace Discord.Utils
         /// <see cref="ValidateButton(string)"/> should be used for url buttons.
         /// </summary>
         /// <param name="url">The URL to validate before sending to Discord.</param>
+        /// <param name="allowAttachments"><see langword="true"/> to allow the <b>attachment://</b> protocol; otherwise <see langword="false"/>.</param>
         /// <exception cref="InvalidOperationException">A URL must include a protocol (http or https).</exception>
         /// <returns>true if URL is valid by our standard, false if null, throws an error upon invalid.</returns>
-        public static bool Validate(string url)
+        public static bool Validate(string url, bool allowAttachments = false)
         {
             if (string.IsNullOrEmpty(url))
                 return false;
-            if (!(url.StartsWith("http://", StringComparison.OrdinalIgnoreCase) || url.StartsWith("https://", StringComparison.OrdinalIgnoreCase)))
-                throw new InvalidOperationException($"The url {url} must include a protocol (either HTTP or HTTPS)");
+            if (!(url.StartsWith("http://", StringComparison.OrdinalIgnoreCase) || url.StartsWith("https://", StringComparison.OrdinalIgnoreCase) || allowAttachments ? url.StartsWith("attachment://", StringComparison.Ordinal) : false))
+                throw new InvalidOperationException($"The url {url} must include a protocol (either {(allowAttachments ? "HTTP, HTTPS, or ATTACHMENT" : "HTTP or HTTPS")})");
             return true;
         }