Thunderbird and iPhone email don't display images inline

[akgrant43]

Describe Your Environment

• ZoneMinder 1.37.45
• How you installed ZoneMinder: PPA
• Ubuntu 22.04.3

Describe the bug
Images, such as added with the `%EI1% macro are added as attachments instead of inline.

To Reproduce

1. Add the %EI1% macro to the alarm email filter and produce an alarm. E.g.:

ZM alarm detected

Monitor: %MN%
Time: %ET%

First alarm image: %EI1%

Highest score image: %EIMA%

Expected behavior
The images are displayed inline (not as attachments).

I'll be adding a PR soon(ish).

Thanks,
Alistair

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