minor improvement on pre-sending file page icon

Session/Meta/Images.xcassets/Session/FileLarge.imageset/Contents.json

@@ -0,0 +1,12 @@
+{
+  "images" : [
+    {
+      "filename" : "doc_large.pdf",
+      "idiom" : "universal"
+    }
+  ],
+  "info" : {
+    "author" : "xcode",
+    "version" : 1
+  }
+}

SignalUtilitiesKit/Media Viewing & Editing/MediaMessageView.swift

@@ -269,7 +269,8 @@ public class MediaMessageView: UIView, OWSAudioPlayerDelegate {
     private func createGenericPreview() {
         var subviews = [UIView]()
 
-        let imageView = createHeroImageView(imageName: "actionsheet_document_black")
+        let imageView = createHeroImageView(imageName: "FileLarge")
+        imageView.contentMode = .center
         subviews.append(imageView)
 
         let fileNameLabel = createFileNameLabel()