With Thunderbird 38.5.1 on Ubuntu 14.04 I see this issue as well.
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https://bugs.launchpad.net/bugs/1249940
Title:
Thunderbird picks incorrect mime type for inline picture when drag-
Gordon, some Apple Mail.app users have told me that they can still see
the images if they rename the suffix. I suppose they have to guess what
the format is, though.
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Status changed to 'Confirmed' because the bug affects multiple users.
** Changed in: thunderbird (Ubuntu)
Status: New = Confirmed
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https://bugs.launchpad.net/bugs/1249940
Title:
I experienced the same problem dragging jpgs to message I was composing.
The resultant attachment is characterized as something.txt, which
confuses many mail readers. It may also be incorrectly encoded. I am
running Thunderbird 24.5.0 on Ubuntu 12.4 on a Dell Vostro (32-bit
Intel) machine.
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