Move media file loading logic to the SDK. (#702)

* Adopt getMediaFile for media previewing.

ElementX/Sources/Other/Extensions/UTType.swift

@@ -0,0 +1,35 @@
+//
+// Copyright 2023 New Vector Ltd
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+
+import UniformTypeIdentifiers
+
+extension UTType {
+    /// Creates a type based on an optional mime type, falling back to a filename when this type is missing or unknown.
+    init?(mimeType: String?, fallbackFilename: String) {
+        guard let mimeType, let type = UTType(mimeType: mimeType) else {
+            self.init(filename: fallbackFilename)
+            return
+        }
+        self = type
+    }
+    
+    /// Creates a type based on a filename.
+    private init?(filename: String) {
+        let components = filename.split(separator: ".")
+        guard components.count > 1, let filenameExtension = components.last else { return nil }
+        self.init(filenameExtension: String(filenameExtension))
+    }
+}