ImportHandler: importing files with unknown extension work again when allowUnknownFileEnds is true

By specification, when settings.allowUnknownFileEnds is true and the user tries
to import a file with an unknown extension (this includes no extension),
Etherpad tries to import it as txt.

This broke in Etherpad 1.8.0, that abruptly terminates the processing with an
UnhandledPromiseRejectionWarning.

This patch restores the intended behaviour, and allows to import as text a file
with an unknown extension (on no extension).

In order to catch the UnhandledPromiseRejectionWarning we had to use
fsp_rename(), which is declared earlier in the code and is promised based
instead of fs.rename(), which is callback based.

Fixes #3710.

src/node/handler/ImportHandler.js

@@ -101,7 +101,7 @@ async function doImport(req, res, padId)
       let oldSrcFile = srcFile;
 
       srcFile = path.join(path.dirname(srcFile), path.basename(srcFile, fileEnding) + ".txt");
-      await fs.rename(oldSrcFile, srcFile);
+      await fsp_rename(oldSrcFile, srcFile);
     } else {
       console.warn("Not allowing unknown file type to be imported", fileEnding);
       throw "uploadFailed";