Edit in JSFiddle

<div class="btn-upload">
    <span>
        <input type="file" id="field-upload" name="files" size="22" class="form-file"/>Choose a file
    </span>
</div>

.btn-upload {
  display: inline-block;
  color: #FFF;
  font-size: 12px;
  line-height: 20px;
  text-align: center;
  text-decoration: none;
  background-color: #4CAE4C;
  position: relative;
  overflow: hidden;
  border-radius: 4px;
}

.btn-upload span {
  display: inline-block;
  padding: 0 16px;
}

.btn-upload input {
  position: absolute;
  top: 0;
  right: 0;
  cursor: pointer;
  font-size: 12px;
  filter: progid:DXImageTransform.Microsoft.Alpha(opacity=0);
  -moz-opacity: 0;
  -khtml-opacity: 0;
  opacity: 0;
  font-size: 21px;
}