What browser is your recipient using?
What happens if they download the file and then change the extension to what it was when you sent it?
What type of file are you sending, and does it have the proper extension?
When they download it there is only one extension to choose and it is some sort of browser extension
It is a pdf file
Did the file have a .pdf extension on it when you sent it? Sometimes, if a file doesn't have an extension, a browser's download dialog will try to add one.
- Have the user download it with whatever extension the browser requires, and then simple rename it with a .pdf extension
- Try a different browser, such as Firefox or Chrome