Create boundary

Hello, I have to put a heart pillow in the designer. If I understand correctly, I have to work with a mask if I want to achieve limitation at the edges. Whenever I try to upload a mask, I get a message saying I would have to use an SVG file. But my file is an SVG file. What am I doing wrong?  

In the appendix my motive. The red line shows the limit that I want to use.