add an extra mime type

Hi, We are a cycling club and we want to make it possible for members to download gpx tours. When I try to import the gpx file, I get the message that this file cannot be uploaden because of security reasons. I tried to do this with the use of two plugins that worked in our former theme from catch themes, but… wat I tried, noting worked. The definition is gpx=application/gpx+xml. I searched the internet for hours. All the comments go in the direction of the plugins I used, but: not result. Is there an advice for? Kind regards Hans Hagmeijer