Fix mimetypes on attachments as some were incorrect. (#7671)

Some file extension can produce invalid MIME types, for example text/txt, image/jpg, image/tif.
Also added htm to the list so that it would not produce application/htm.

services/src/main/java/org/fao/geonet/api/records/attachments/AttachmentsApi.java

@@ -111,15 +111,22 @@ public static String getFileContentType(Path file) throws IOException {
                 case "png":
                 case "gif":
                 case "bmp":
+                    contentType = "image/" + ext;
+                    break;
                 case "tif":
                 case "tiff":
+                    contentType = "image/tiff";
+                    break;
                 case "jpg":
                 case "jpeg":
-                    contentType = "image/" + ext;
+                    contentType = "image/jpeg";
                     break;
                 case "txt":
+                    contentType = "text/plain";
+                    break;
+                case "htm":
                 case "html":
-                    contentType = "text/" + ext;
+                    contentType = "text/html";
                     break;
                 default:
                     contentType = "application/" + ext;