MemberPress: Add Thumbnail Support for MP Downloads

By default, the MemberPress Downloads add-on doesn't support thumbnails or featured images for downloadable files. This code snippet enables thumbnail support for files in the MemberPress Downloads add-on, allowing featured images to be added to downloadable files and displayed throughout the website.

The code adds native WordPress thumbnail support to the MP Downloads custom post type (mpdl-file), allowing the following:
- Upload featured images for each downloadable file
- Display these thumbnails alongside your downloads in listings
- Use standard WordPress thumbnail functions to access these images

Once the code is active on the website, thumbnail can be added to each file following these steps:
1. Navigate to Dashboard > MP Downloads > Files.
2. Edit any file to access the Featured Image panel in the right sidebar.
3. Upload or select an image to set as the thumbnail for the file.

Customization Options:
To display the thumbnails with your downloads, you'll need to customize the MP Downloads templates. You can do this by:
1. Locating the template files in your plugin directory:
- memberpress-downloads/app/views/files/file_link.php
- memberpress-downloads/app/views/files/file_links.php
2. Creating copies of these files in your theme directory to override them.
3. Adding the WordPress thumbnail function to display the image

// Display thumbnail for MP Downloads files
$thumbnail = get_the_post_thumbnail($file->ID, 'thumbnail');
if ($thumbnail) {
echo $thumbnail;
}

You can place this code where you want the thumbnail to appear in the template. The thumbnail parameter specifies the image size. You can change this to other registered image sizes, such as medium, large, or custom sizes.