A look at Outlook's 'extra' attachment bug

Outlook can get attached to attachments

Microsoft Outlook
Credit: IDG

The problem

Occasionally Outlook emails with attached images may also include extra files named ATT00001, ATT00002... and so on. These "ATT" files are also "thrown in" with images inside zips created using Outlook's Download all as zip option. 

Although this bug is rare, after multiple attempts I was able to recreate the bug using a Windows Phone to send multiple image attachments via a Microsoft Live account. The images included landscape and portrait screenshots and a photo taken with the stock Windows Phone Camera app.

And -- with a little help from a hex editor -- I found other nuggets of data about the extra file attachments.

These are steps I used to recreate this bug.

Step 1

Select a group of photos and screenshots saved in a variety of orientations and sizes (landscape, portrait, large, small, etc) and select Share.

Windows Phone Select images to share Stephen Glasskeys

Step 2

Share using Microsoft Live email.

Share using Microsoft Live email Stephen Glasskeys

Step 3

Open your Outlook.com email account and click Sent messages. Select the image file and click Download all as zip

Outlook Download as zip Stephen Glasskeys

Step 4

Extract the zip or drill down with Explorer. If you do not see the extra "ATT" files, you'll need to try again to create this bug using a different set of images. 

Extracted zip with extra ATT files Stephen Glasskeys

Step 5

Open the ATT00001 file with the HxD hex editor. The hex dump reveals the file is saved in JPEG File Interchange Format (JFIF) format...

HxD dump of ATT00001 file Stephen Glasskeys

....which means a .jpg extension should to be appended to each of the ATT filenames.

Append jpg extension to ATT files Stephen Glasskeys

Step 6

Opening a newly renamed ATT file in preview shows a small thumbnail version of one of the original image attachments.

ATT00001 thumbnail image Stephen Glasskeys

Finally, we can easily compare thumbnails to original images using Windows Explorer's Properties feature.

Windows Explorer file properties, details tab Stephen Glasskeys

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