Upload fails when user drags and drops attachment from email client

I found a free solution to your drag and drop problem: https://tonyfederer.github.io/OutlookFileDrag/

From the website:

When you try to drag and drop from Outlook, Outlook correctly identifies the format as virtual files (CFSTR_FILEDESCRIPTORW) since the files do not exist directly on disk. Instead, they are contained in a PST file, OST file, or on an Exchange server.

However, many applications do not support, such as web browers and most .NET/ Java applications.

To work around this issue, Outlook File Drag hooks the Outlook drag and drop process and adds support for physical files (CF_HDROP). When the application asks for the physical files, the files are saved to a temp folder.

I looked at https://outlook2web.com/ and https://www.wilutions.info/ddaddin-download.html

but the free one works fine, and it's open source!

June 2020 Update: Chrome and Edge can now interface with the latest Outlook, FireFox still has an open ticket though: https://bugzilla.mozilla.org/show_bug.cgi?id=580928

Most browsers are only expecting the actual file drag/drop format (CF_HDROP), but when dragging from Outlook, there is no file on the filesystem. What you get is the CF_FILEDESCRIPTOR and CF_FILECONTENTS formats. No browser that I know of (not even IE), knows how to handle that.

FWIW dragging from client to browser now works with Microsoft Edge and Outlook.

Tested with Outlook 2016 and Edge 42.17134.1.0