下载类书源文件名：文件链接支持type字段

https://www.example.com/aaaaaa, {type: "txt"}

书名.txt

app/src/main/java/io/legado/app/ui/book/info/BookInfoViewModel.kt

@@ -241,9 +241,10 @@ class BookInfoViewModel(application: Application) : BaseViewModel(application) {
     ) {
         execute(scope) {
             webFiles.clear()
-            val fileName = "${book.name} 作者：${book.author}"
+            val fileNameExcludeExtension = if (book.author.isBlank()) book.name else "${book.name} 作者：${book.author}"
             book.downloadUrls!!.map {
-                val mFileName = UrlUtil.getFileName(AnalyzeUrl(it, source = bookSource)) ?: fileName
+                val analyzeUrl = AnalyzeUrl(it, source = bookSource)
+                val mFileName = UrlUtil.getFileName(analyzeUrl) ?: "${fileNameExcludeExtension}.${analyzeUrl.type}"
                 WebFile(it, mFileName)
             }
         }.onError {