Recognise dragged iOS screenshots as images instead of raw data

When dragging the floating screenshot preview into a room, iOS hands us
a provider whose `suggestedName` is e.g. `Screenshot 2026-05-22 at
14.49.40`. `NSString.pathExtension` returns `"40"`, so the previous
`!pathExtension.isEmpty` check treated the timestamp's trailing segment
as a real extension. The file got written without `.jpg`/`.png`, and the
`MediaUploadingPreprocessor` then fell back to `application/octet-stream`
which is why screenshots showed up as a "data" tile.

ElementX/Sources/Other/Extensions/NSItemProvider.swift

@@ -196,8 +196,15 @@ extension NSItemProvider {
     }
 }
 
-private extension NSString {
+extension NSString {
+    /// `NSString.pathExtension` returns the trailing dot-segment regardless of whether
+    /// it's a real extension, and `UTType(filenameExtension:)` synthesises a `dyn.*`
+    /// placeholder for unknown ones instead of returning nil — so check both.
     var hasPathExtension: Bool {
-        !pathExtension.isEmpty
+        guard !pathExtension.isEmpty,
+              let type = UTType(filenameExtension: pathExtension) else {
+            return false
+        }
+        return !type.isDynamic
     }
 }