Fix inconsistent capitalisation of downloaded files

Fixes JabRef#6115

Author: stefan-kolb

CHANGELOG.md

@@ -20,6 +20,8 @@ Note that this project **does not** adhere to [Semantic Versioning](http://semve
 ### Fixed
 
 - We fixed an issue where opening a library from the recent libraries menu was not possible. [#5939](https://github.com/JabRef/jabref/issues/5939)
+- We fixed an issue where opening a library from the recent libraries menu was not possible [#5939](https://github.com/JabRef/jabref/issues/5939)
+- We fixed an issue with inconsistent capitalization of file extensions when downloading files [#6115](https://github.com/JabRef/jabref/issues/6115)
 
 ### Removed
 

src/main/java/org/jabref/gui/fieldeditors/LinkedFileViewModel.java

@@ -421,10 +421,10 @@ public BackgroundTask<Path> prepareDownloadTask(Path targetDirectory, URLDownloa
         BackgroundTask<Path> downloadTask = BackgroundTask
                 .wrap(() -> {
                     Optional<ExternalFileType> suggestedType = inferFileType(urlDownload);
-                    String suggestedTypeName = suggestedType.orElse(StandardExternalFileType.PDF).getName();
+                    ExternalFileType externalFileType = suggestedType.orElse(StandardExternalFileType.PDF);
+                    String suggestedTypeName = externalFileType.getName();
                     linkedFile.setFileType(suggestedTypeName);
-
-                    String suggestedName = linkedFileHandler.getSuggestedFileName(suggestedTypeName);
+                    String suggestedName = linkedFileHandler.getSuggestedFileName(externalFileType.getExtension());
                     return targetDirectory.resolve(suggestedName);
                 })
                 .then(destination -> new FileDownloadTask(urlDownload.getSource(), destination))