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.btn-file {
    position: relative;
    overflow: hidden;
}
.btn-file input[type=file] {
    position: absolute;
    top: 0;
    right: 0;
    min-width: 100%;
    min-height: 100%;
    font-size: 100px;
    text-align: right;
    filter: alpha(opacity=0);
    opacity: 0;
    outline: none;
    background: white;
    cursor: inherit;
    display: block;
}

<h3>Upload new Images</h3>
<form method=post enctype=multipart/form-data>
  <div class='form-group'>
    <label class="btn btn-default btn-file">
        Browse <input type="file" name=file style="display: none;">
    </label>
    <button type="submit" class="btn btn-success">Upload</button>
  </div>
</form>