Thunderbird does not have a 'Send to' option, so it does not sound like that description is from something in Thunderbird. It sounds more like an option in Windows explorer when you right click a file/image to see a drop down menu. If that is the case, then there has been plenty of reports in Microsoft forums that Microsoft has caused issues for the 'Send to' function and has not chosen to fix anything. I get various results and not always as expected. Currently when I use the 'Send to' > 'Thunderbird' option instead of attaching to Thunderbird it actaully opens the eg: image in Photos or if a pdf it open in Adobe Reader etc. So far, I've not found a solution to this weirdness, so use either of the two methods below To attach a document in Thunderbird there are two easy methods. In Thunderbird * Click on '+New Message' * Click on 'Attach' * Locate image and click on 'Open' Image should get attached to the message you wish to send. OR In Thunderbird * Click on '+New Message' Then using file explorer locate the file you want to attach - you may already have open :) Left click on hold down on file icon and drag over to the '+New Message' aka 'Write' window where you will get a visual prompt to either attach inline or as attachment - just release mouse to drop on the option on left or right. The one on the right will be 'Attachment'.