Your Contact Form 7 is using
accept=".pdf" on the file input HTML.
This instructs browsers that know how, to only show files which have a file name matching the pattern. Ideally your contact form would also specify the PDF mime type which would give slighty wider browser support, especially on mobile:
accept=".pdf,application/pdf". However, even that won't work with Microsoft Edge.
It is a known issue that Microsoft Edge doesn't really support the accept attribute for file input. In Microsoft Edge the
All files (*) drop down contains the filter, but it is never selected by default. Users have to manually select it from the drop down to get the filter applied. Can I Use reports that Edge does not support this feature even though Microsoft Internet Explorer does.
On StackOverflow Limit file format when using
<input type=“file”>? says that this is a browser client side feature that is easy to override or hack. If it is important that users can only upload PDF files, your server side controller needs to enforce that.