This didn't make it to the recent build, but I'd like to improve this part as well.

When you attach a file to composer, it's loaded into memory. You can as well delete the physical file from the disk and Wino can still send the attachment. Therefore opening the file for preview requires content to be saved to temporary folder in case of the file is deleted.

I'll have it implemented for the next version.

There's also the issue that this particular area of the "button" activated the delete action instead of the open action.