Handle gotosocial gif attachments

Pachyderm/Model/Attachment.swift

@@ -56,6 +56,23 @@ extension Attachment {
         case gifv
         case audio
         case unknown
+        
+        public init(from decoder: Decoder) throws {
+            let container = try decoder.singleValueContainer()
+            switch try container.decode(String.self) {
+            // gotosocial uses "gif" for gif images
+            case "image", "gif":
+                self = .image
+            case "video":
+                self = .video
+            case "gifv":
+                self = .gifv
+            case "audio":
+                self = .audio
+            default:
+                self = .unknown
+            }
+        }
     }
 }