[SSK] Prefer to deduce the MIME type from the file extension using lookup, not the UTI type.

[SSK] Try to deduce attachment MIME type from the file extension if possible.

// FREEBIE

Podfile

@@ -5,8 +5,8 @@ target 'Signal' do
     pod 'SocketRocket',               :git => 'https://github.com/facebook/SocketRocket.git'
     pod 'AxolotlKit',                 git: 'https://github.com/WhisperSystems/SignalProtocolKit.git'
     #pod 'AxolotlKit',                 path: '../SignalProtocolKit'
-    #pod 'SignalServiceKit',           git: 'https://github.com/WhisperSystems/SignalServiceKit.git'
-    pod 'SignalServiceKit',           path: '../SignalServiceKit'
+    pod 'SignalServiceKit',           git: 'https://github.com/WhisperSystems/SignalServiceKit.git'
+    #pod 'SignalServiceKit',           path: '../SignalServiceKit'
     pod 'OpenSSL'
     pod 'JSQMessagesViewController',  git: 'https://github.com/WhisperSystems/JSQMessagesViewController.git', branch: 'mkirk/position-edit-menu'
     #pod 'JSQMessagesViewController'   path: '../JSQMessagesViewController'

Podfile.lock

@@ -114,7 +114,7 @@ DEPENDENCIES:
   - JSQMessagesViewController (from `https://github.com/WhisperSystems/JSQMessagesViewController.git`, branch `mkirk/position-edit-menu`)
   - OpenSSL
   - PureLayout
-  - SignalServiceKit (from `../SignalServiceKit`)
+  - SignalServiceKit (from `https://github.com/WhisperSystems/SignalServiceKit.git`)
   - SocketRocket (from `https://github.com/facebook/SocketRocket.git`)
 
 EXTERNAL SOURCES:
@@ -124,7 +124,7 @@ EXTERNAL SOURCES:
     :branch: mkirk/position-edit-menu
     :git: https://github.com/WhisperSystems/JSQMessagesViewController.git
   SignalServiceKit:
-    :path: ../SignalServiceKit
+    :git: https://github.com/WhisperSystems/SignalServiceKit.git
   SocketRocket:
     :git: https://github.com/facebook/SocketRocket.git
 
@@ -135,6 +135,9 @@ CHECKOUT OPTIONS:
   JSQMessagesViewController:
     :commit: 7054e4b13ee5bcd6d524adb6dc9a726e8c466308
     :git: https://github.com/WhisperSystems/JSQMessagesViewController.git
+  SignalServiceKit:
+    :commit: 92de9a5e72f12dc242beaba8b62388207de19d9b
+    :git: https://github.com/WhisperSystems/SignalServiceKit.git
   SocketRocket:
     :commit: 877ac7438be3ad0b45ef5ca3969574e4b97112bf
     :git: https://github.com/facebook/SocketRocket.git
@@ -161,6 +164,6 @@ SPEC CHECKSUMS:
   UnionFind: c33be5adb12983981d6e827ea94fc7f9e370f52d
   YapDatabase: b1e43555a34a5298e23a045be96817a5ef0da58f
 
-PODFILE CHECKSUM: 74ea2172e28ade89124e817d3eae14753f1e9bbf
+PODFILE CHECKSUM: 549de6756fe8eab98647be8561b3988361f62e85
 
 COCOAPODS: 1.2.1