Option to list only emails with attachments sorted by; images, docs, PDFs, etc. is a killer feature.
How many of us bother to file away every attachment we receive? How can you search for an attachment that you can't remember who sent it to you? How can you search for an attachment that has a cryptic file name? How can you find an attachment that was opened in an app that you can't recall which you used? An option that can use a pre-configured filter that lists the attachments by docs, jpg, ppt, PDF, videos, spreadsheets, would be a killer feature.
Tony Di Natale commented
Take a look at how Mailcloud has the pre-configured filters for attachments. The feature is awesome for easily locating ALL attachments.