Make file extension work and remove some unneeded stuff

lib/export-to-csv.js

@@ -8,7 +8,8 @@ export default function makeDataURI() {
 
   const csv = transformToCsv(cycleDaysSortedByDate)
   const encoded = Base64.encodeURI(csv)
-  return `data:text/csv;base64,${encoded}`
+  // this is the MIME type android/libcore/MimeUtils expects, so we oblige
+  return `data:text/comma-separated-values;base64,${encoded}`
 }
 
 function transformToCsv(cycleDays) {