Custom Map Format's extension can't include dot on MacOS

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ENV:
MacOS: 10.15
Tiled: 1.3RC

Fllow http://doc.mapeditor.org/en/latest/reference/scripting/#script-registermapformat:
```javascript
var customMapFormat = {
name: "Custom map format",
extension: "custom.txt",

write: function(map, fileName) {
var m = {
width: map.width,
height: map.height,
layers: []
};

for (var i = 0; i < map.layerCount; ++i) {
var layer = map.layerAt(i);
if (layer.isTileLayer) {
var rows = [];
for (y = 0; y < layer.height; ++y) {
var row = [];
for (x = 0; x < layer.width; ++x)
row.push(layer.cellAt(x, y).tileId);
rows.push(row);
}
m.layers.push(rows);
}
}

return JSON.stringify(m);
},
}

tiled.registerMapFormat("custom", customMapFormat)
```

When I select the custom format in the export dialog, the export box is hung up.

Start by reproducing the export-dialog hang on macOS using the documented registerMapFormat example with extension "custom.txt". Trace the registerMapFormat and export-dialog path; done means selecting the custom format no longer hangs and the map exports with the dotted extension.

Written by the indexing model from the issue text.