title: Filter by mimetype or extention in native Android file picker
body: <p>In Android, one can use the <code>ACTION_OPEN_DOCUMENT</code> Intent to <strong>open the native file picker</strong> and select for example an <code>.mp4</code> file. This is achived by setting the mime type to <code>video/mp4</code> using the following code:</p>
  
  <pre><code>public static void pickFile(Context mContext, int REQUEST_CODE) {
          Intent intent = new Intent(Intent.ACTION_OPEN_DOCUMENT);
          intent.addCategory(Intent.CATEGORY_OPENABLE);
          intent.setType("video/mp4");  
          ((Activity) mContext).startActivityForResult(intent, REQUEST_CODE);
      }
  </code></pre>
  
  <p>In my project, I want to pick a custom extension file whose mime type is not known in android's <a href="https://developer.android.com/reference/android/webkit/MimeTypeMap" rel="noreferrer">MimeTypeMap</a> like for example .qgs or .dcm files.</p>
  
  <p>To solve this I see two possibilities that we, so far, failed to implement:</p>
  
  <ol>
  <li>filter by extension in the <code>ACTION_OPEN_DOCUMENT</code> Intent</li>
  <li>register a new mime type to android so that it can be used with the <code>ACTION_OPEN_DOCUMENT</code> Intent</li>
  </ol>
  
  <p><strong>is either of those options doable and how?</strong>
  or are there other approaches I missed without coding my own file picker?</p>