23 fix contenttype attachment

[Miksus]

See: issue 23

[codecov-commenter]

Codecov Report

Merging #24 (8aec193) into master (f19d4ca) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #24   +/-   ##
=======================================
  Coverage   99.59%   99.59%           
=======================================
  Files          12       12           
  Lines         494      494           
=======================================
  Hits          492      492           
  Misses          2        2           

Impacted Files	Coverage Δ
redmail/email/attachment.py	100.00% <100.00%> (ø)
redmail/email/body.py	100.00% <0.00%> (ø)

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[Miksus]

This works for without bodies, with text body, with HTML body and with text and HTML body (tested sent Gmail, viewed Gmail & Windows Mail app).

Seems at least the Windows Mail app does not like an email with an attachment and without text and HTML bodies (does not show the email has an attachment).