Add image file accept to openapi

Jellyfin.Api/Attributes/AcceptsImageFileAttribute.cs

@@ -0,0 +1,18 @@
+namespace Jellyfin.Api.Attributes
+{
+    /// <summary>
+    /// Produces file attribute of "image/*".
+    /// </summary>
+    public class AcceptsImageFileAttribute : AcceptsFileAttribute
+    {
+        private const string ContentType = "image/*";
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="AcceptsImageFileAttribute"/> class.
+        /// </summary>
+        public AcceptsImageFileAttribute()
+            : base(ContentType)
+        {
+        }
+    }
+}