body {
  text-align: center;
  font-size: 25px;
}

.file-input.custom{
  /*your custom styles*/
  background: darkgreen;
  width: 100%;
  height: 75px;
  bottom: 0px;
  left: 0px;
  position: fixed;
}

.file-input{
  pointer-events: none;
  position: relative;
  overflow: hidden;
}
.file-input > * {
  pointer-events: none;
}
.file-input > input[type="file"]{
  position: absolute;
  top: 0;
  right: 0;
  bottom: 0;
  left: 0;
  opacity: 0;
  pointer-events: all;
  cursor: pointer;
  height: 100%;
  width: 100%;
}