CSS: How to make custom file upload button take full width (button works)

Question

I am new to CSS and hope someone here can help me with this.

I am trying to apply a simple custom style to a file upload button (as part of an HTML form) to make it look similar to other buttons on my page and to get a similar look cross-browser.

So far I have the following which works as intended. My only problem now is that I would like the button to take the full width of its parent div (in my case this will span across 9/12 ('col-9') of the page).

I tried adding width: 100%; to the CSS but then the button doesn't work anymore.

My HTML:

<div class="col-3 frmCaption">Attachments:</div>
<div class="col-9">
    <div class="customUpload btnUpload btnM">
        <span>Upload files</span>
        <input type="file" class="upload" />
    </div>
</div>

My CSS:

.btnDefault, .btnUpload {
    background-color: #FFFFFF;
    border: 1px solid #CCCCCC;
    color: #333333;
    cursor: pointer;
    font-weight: 400;
    display: inline-block;
    padding: 6px 12px;
    text-align: center;
    text-decoration: none;
    vertical-align: middle;
}       
.btnDefault:focus, .btnDefault:hover, .btnUpload:focus, .btnUpload:hover {
    background-color: #E6E6E6;
}
.btnM {
    border-radius: 4px;
    font-size: 14px;
    padding: 6px 12px;
}
.customUpload {
    overflow: hidden;
    position: relative;
}
.customUpload input.upload {
    cursor: pointer;
    margin: 0;
    opacity: 0;
    filter: alpha(opacity=0);
    padding: 0;
    position: absolute;
    right: 0;
    top: 0;
}

Answer 1

To style input elements, you need to actually style its label element.

From MDN,

The HTML Label Element () represents a caption for an item in a user interface. It can be associated with a control either by placing the control element inside the element, or by using the for attribute. Such a control is called the labeled control of the label element.

So, whenever you click a label, the attached input gets triggered.

So, just wrap the input element in a label instead of a div and stretch as much as you want. That will fix your issue.

.btnDefault,
.btnUpload {
  background-color: #FFFFFF;
  border: 1px solid #CCCCCC;
  color: #333333;
  cursor: pointer;
  font-weight: 400;
  display: inline-block;
  padding: 6px 12px;
  text-align: center;
  text-decoration: none;
  vertical-align: middle;
}
.btnDefault:focus,
.btnDefault:hover,
.btnUpload:focus,
.btnUpload:hover {
  background-color: #E6E6E6;
}
.btnM {
  border-radius: 4px;
  font-size: 14px;
  padding: 6px 12px;
}
.customUpload {
  overflow: hidden;
  position: relative;
  display: block;
}
.customUpload input.upload {
  cursor: pointer;
  margin: 0;
  opacity: 0;
  filter: alpha(opacity=0);
  padding: 0;
  position: absolute;
  right: 0;
  top: 0;
}

<div class="col-3 frmCaption">Attachments:</div>
<div class="col-9">
  <label class="customUpload btnUpload btnM"> <span>Upload files</span>

    <input type="file" class="upload" />
  </label>
</div>

Working Fiddle