fix: support non-PNG embedded images as dict

[Miksus]

Fix to support also other than just PNG formats passed as dict, like:

email.send(
    subject="Example",
    receivers=[...],
    html="<h1>Example</h1> {{ my_image }}",
    body_images={
        'my_image': {
            'content': b'<image as bytes>',
            'maintype': 'image',
            'subtype': 'jpeg',
        }
    }
)

[Miksus]

Probably good to have one JPEG test as well before merging.

[codecov-commenter]

Codecov Report

Merging #16 (422af9d) into master (f19d4ca) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #16      +/-   ##
==========================================
- Coverage   99.59%   99.59%   -0.01%     
==========================================
  Files          12       12              
  Lines         494      492       -2     
==========================================
- Hits          492      490       -2     
  Misses          2        2              

Impacted Files	Coverage Δ
redmail/email/body.py	100.00% <100.00%> (ø)

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[Miksus]

Also, now one does not need to specify maintype as that probably should always be "image".