Write a Content-Disposition header for attach files.

src/main/scala/gitbucket/core/controller/IssuesController.scala

@@ -332,6 +332,7 @@ trait IssuesControllerBase extends ControllerBase {
     (Directory.getAttachedDir(repository.owner, repository.name) match {
       case dir if(dir.exists && dir.isDirectory) =>
         dir.listFiles.find(_.getName.startsWith(params("file") + ".")).map { file =>
+          response.setHeader("Content-Disposition", f"""inline; filename=${file.getName}""")
           RawData(FileUtil.getMimeType(file.getName), file)
         }
       case _ => None