"AbortError: The operation was aborted. " on uploading multiple image files from the clipboard

# Steps:

1. Start to update an issue.
2. Copy an embedded image in a mail.
3. Paste it to the "notes" field.
4. Repeat 2-3 for more two images.
5. Click "OK".

# Expected:

Images are successfully uploaded.

# Actual:

"AbortError: The operation was aborted. " is reported.
It is returned by the server side.


# Environment

* Thunderbird 78.4.2
* RedThunderMIneBird Plus 2.1.1


