I am working on sending an email with the opportunity files based on certain criteria. I am able to send an email with the 'versionData' as an email body which is queried from 'Contentversion'. 
Problem: When i try to open the attachment from email it says 'Preview not available' and also i am not able to open the downloaded attachment too.
Please find the code below and let me know where I am going wrong. Thanks!

List<id> ContentDocumentids = new List<id>();

for(contentDocumentLink CDLink : [SELECT LinkedEntityid, ContentDocumentid FROM contentDocumentLink WHERE LinkedEntityid=:'Object id'])
                {
                   ContentDocumentids.add(CDLink.ContentDocumentid);  
                }
                for ( ContentVersion cversion : [SELECT title, 
                                                        PathOnClient, 
                                                        versiondata 
                                                  FROM contentversion 
                                                  WHERE ContentDocumentId IN :ContentDocumentids  
                                                   AND title LIKE 'WOCF%'])
                 {
                  blob WOCFbody = cversion.versiondata;
                  system.debug('body : '+WOCFbody+'-----------'+cversion.title);
                  Messaging.Emailfileattachment efa1 = new Messaging.Emailfileattachment();
                  efa.setFileName(war2.opportunity__r.name+'-'+cversion.title);
                  efa.setBody(WOCFbody);
                  fileAttachments.add(efa); 
                 }