allow pasting attachments from keyboard

Closes #55

android/app/src/main/AndroidManifest.xml

@@ -51,6 +51,16 @@
         <meta-data
             android:name="flutterEmbedding"
             android:value="2" />
+        <!-- Needed by pasteboard, for pasting files from the clipboard -->
+        <provider
+            android:name="androidx.core.content.FileProvider"
+            android:authorities="${applicationId}.provider"
+            android:exported="false"
+            android:grantUriPermissions="true">
+            <meta-data
+                android:name="android.support.FILE_PROVIDER_PATHS"
+                android:resource="@xml/provider_paths" />
+        </provider>
     </application>
     <!-- Required to query activities that can process text, see:
          https://developer.android.com/training/package-visibility and

android/app/src/main/res/xml/provider_paths.xml

@@ -0,0 +1,6 @@
+<?xml version="1.0" encoding="utf-8"?>
+<paths xmlns:android="http://schemas.android.com/apk/res/android">
+    <external-path
+        name="external_files"
+        path="." />
+</paths>