fix(Android <input />): Support file extensions in file input accept attributes (#357)

The current implementation doesn't support a list of file extensions in the accept attribute (e.g. `<input type="file" accept=".jpg, .png">`) while that is a valid value per the HTML spec. I've updated the implementation to convert any file extensions to mime types before we set the type(s) on the intent to rectify the issue. In addition I've updated the `acceptsImages` and `acceptsVideo` methods to handle file extensions as well.

android/src/main/java/com/reactnativecommunity/webview/RNCWebViewModule.java

 import android.support.v4.content.ContextCompat;
 import android.support.v4.content.FileProvider;
 import android.util.Log;
+import android.webkit.MimeTypeMap;
 import android.webkit.ValueCallback;
 import android.webkit.WebChromeClient;
 import android.widget.Toast;
     if (acceptTypes.isEmpty()) {
       _acceptTypes = DEFAULT_MIME_TYPES;
     }
+    if (acceptTypes.matches("\\.\\w+")) {
+      _acceptTypes = getMimeTypeFromExtension(acceptTypes.replace(".", ""));
+    }
     Intent intent = new Intent(Intent.ACTION_GET_CONTENT);
     intent.addCategory(Intent.CATEGORY_OPENABLE);
     intent.setType(_acceptTypes);
   }
 
   private Boolean acceptsImages(String types) {
-    return types.isEmpty() || types.toLowerCase().contains("image");
+    String mimeType = types;
+    if (types.matches("\\.\\w+")) {
+        mimeType = getMimeTypeFromExtension(types.replace(".", ""));
+    }
+    return mimeType.isEmpty() || mimeType.toLowerCase().contains("image");
   }
   private Boolean acceptsImages(String[] types) {
-    return isArrayEmpty(types) || arrayContainsString(types, "image");
+    String[] mimeTypes = getAcceptedMimeType(types);
+    return isArrayEmpty(mimeTypes) || arrayContainsString(mimeTypes, "image");
   }
 
   private Boolean acceptsVideo(String types) {
-    return types.isEmpty() || types.toLowerCase().contains("video");
+    String mimeType = types;
+    if (types.matches("\\.\\w+")) {
+        mimeType = getMimeTypeFromExtension(types.replace(".", ""));
+    }
+    return mimeType.isEmpty() || mimeType.toLowerCase().contains("video");
   }
   private Boolean acceptsVideo(String[] types) {
-    return isArrayEmpty(types) || arrayContainsString(types, "video");
+    String[] mimeTypes = getAcceptedMimeType(types);
+    return isArrayEmpty(mimeTypes) || arrayContainsString(mimeTypes, "video");
   }
 
   private Boolean arrayContainsString(String[] array, String pattern){
     if (isArrayEmpty(types)) {
         return new String[]{DEFAULT_MIME_TYPES};
     }
-    return types;
+    String[] mimeTypes = new String[types.length];
+    for (int i = 0; i < types.length; i++) {
+        String t = types[i];
+        // convert file extensions to mime types
+        if (t.matches("\\.\\w+")) {
+            String mimeType = getMimeTypeFromExtension(t.replace(".", ""));
+            mimeTypes[i] = mimeType;
+        } else {
+            mimeTypes[i] = t;
+        }
+    }
+    return mimeTypes;
+  }
+
+  private String getMimeTypeFromExtension(String extension) {
+    String type = null;
+    if (extension != null) {
+        type = MimeTypeMap.getSingleton().getMimeTypeFromExtension(extension);
+    }
+    return type;
   }
 
   private Uri getOutputUri(String intentType) {