The master parts catalog shows the fastener to be part # 55927-S, which it then says is a "No. 8-18 x 1/2” screw". From what I can see it is a normal sheet metal screw. But you really need something with more surface area if you are installing the soft garage door seal, so I'd use something like this and put it through the large plastic washers that come with the seal:
Gary, AKA "Gary fellow":
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Dad's: '81 F150 Ranger XLT 4x4: Down for restomod: Full-roller "stroked 351M" w/Trick Flow heads & intake, EEC-V SEFI/E4OD/3.50 gears w/Kevlar clutches
Blue: 2015 F150 Platinum 4x4 SuperCrew wearing Blue Jeans & sporting a 3.5L EB & Max Tow
Big Blue: 1985 F250HD 4x4: 460/ZF5/3.55's, D60 w/Ox locker & 10.25 Sterling/Trutrac, Blue Top & Borgeson, & EEC-V MAF/SEFI