import usercss files drag'n'dropped into the manage page

install-usercss/install-usercss.js

@@ -276,7 +276,7 @@
           data.version,
         ]))
       ).then(ok => ok &&
-        BG.usercssHelper.save(BG.deepCopy(Object.assign(style, {reason: 'update'})))
+        BG.usercssHelper.save(BG.deepCopy(Object.assign(style, dup && {reason: 'update'})))
           .then(r => install(deepCopy(r)))
           .catch(err => messageBox.alert(t('styleInstallFailed', err)))
       );

js/usercss.js

@@ -393,6 +393,7 @@ var usercss = (() => {
     };
 
     const style = {
+      reason: 'install',
       enabled: true,
       sourceCode,
       sections: [],

manage/import-export.js

@@ -34,7 +34,17 @@ function importFromFile({fileTypeFilter, file} = {}) {
         const fReader = new FileReader();
         fReader.onloadend = event => {
           fileInput.remove();
-          importFromString(event.target.result).then(numStyles => {
+          const text = event.target.result;
+          const maybeUsercss = !/^[\s\r\n]*\[/.test(text) &&
+            (text.includes('==UserStyle==') || /==UserStyle==/i.test(text));
+          (!maybeUsercss ?
+            importFromString(text) :
+            getOwnTab().then(tab => {
+              tab.url = URL.createObjectURL(new Blob([text], {type: 'text/css'}));
+              return BG.usercssHelper.openInstallPage(tab, {direct: true})
+                .then(() => URL.revokeObjectURL(tab.url));
+            })
+          ).then(numStyles => {
             document.body.style.cursor = '';
             resolve(numStyles);
           });