Okay Ignore my previous suggestion it seems that no longer works try this, I tested it on my side and it works:
Step 1 click into the form the inside the attachment datacard or a new custom one either works insert a button
Step 2 on this new buttons onselect add Reset(DataCardValue7) (Assumption made by your post that the attachment control is called DataCardValue7
(image above for step 1 and 2)

Step 3 make it invisible by changing the visible property
Step 4 add a new button anywhere you want to reset it and on it's select write Select(Insert the name of the button we put inside the form)
(Image relating to final step above)