Detect mime type when picking a file

The file picker will now try to detect the mime type of the selected file.
If it cannot be detected, it will fallback to `application/octet-stream`.

libraries/mediapickers/api/src/main/kotlin/io/element/android/libraries/mediapickers/api/PickerProvider.kt

@@ -25,7 +25,7 @@ interface PickerProvider {
     @Composable
     fun registerFilePicker(
         mimeType: String,
-        onResult: (Uri?) -> Unit
+        onResult: (uri: Uri?, mimeType: String?) -> Unit,
     ): PickerLauncher<String, Uri?>
 
     @Composable