Takayuki Miyoshi on "[Plugin: Contact Form 7] The upload button doesn't show"

Your theme is making it transparent.

http://www.vetpetson.com.br/site/wp-content/themes/CherryFramework/css/style.css
Line #2241

.wpcf7-form input[type="file"] {
  position: relative;
  width: auto;
  height: 100%;
  opacity: 0;
  filter: progid:DXImageTransform.Microsoft.Alpha(opacity=0);
  cursor: pointer;
  -moz-opacity: 0;
}

Removing "opacity: 0;" line (maybe "-moz-opacity: 0;" as well) will make it show up.