Upgrade to 6.1 failed on Document Library migration when using AdvancedFileSystemHook and generating thumbnails

//This is UpgradeDocumentLibrary.java line 448
// NOT SETTING THE extension PROPERTY
dlFileVersion.setFileVersionId(fileVersionId);
dlFileVersion.setUserId(userId);
dlFileVersion.setFileEntryId(fileEntryId);
dlFileVersion.setMimeType(mimeType);
dlFileVersion.setVersion(version);

//This is ImageProcessor.java line 280
// Using the getExtension() method from the FileVersion created before, but it returns null.
private String _getType(FileVersion fileVersion) {
		String type = fileVersion.getExtension();

		if (type.equals("jpeg")) {
			type = "jpg";
		}

		return type;
	}

//This is DLPreviewableProcessor.java line 331
// Generating the path for the thumbnail using the concat(StringPool.PERIOD).concat(type) but type is null because the previous method returns null
protected String getThumbnailFilePath(
		FileVersion fileVersion, String type) {

		return getPathSegment(fileVersion, false).concat(
			StringPool.PERIOD).concat(type);
	}