in the property accept  specify the type of the desired file as "application/pdf"

 

I allow images, pdf and msword files in my upload as in (suppose could be a static value as well)

<xp:this.accept><![CDATA[${javascript:"image/gif,image/jpeg,image/pjpeg,image/png,image/bmp,image/tiff,application/pdf,application/msword,application/vnd.openxmlformats-officedocument.wordprocessingml.document"}]]></xp:this.accept>   

Amazing, thank you both, it works.

 

I see that it just blanks out the upload field, with no word of explanation; I wonder if there's a way to communicate with the user why their upload got blanked out.