Wrong content_type_t when adding an attachment

[david-antiteum]

If an email has content in HTML and then an attachment, the content losses the type.

• In message::attach it overwrites the attribute _content_type
• In message::format it sets the content as text/plain

[niclar]

@karastojko this is still an issue

[karastojko]

@niclar
This PR is not merged because there was another issue not solved with this change. There was a commit which tries to fix two defects. Did you try
void attach(const std::istream&, const std::string&, media_type_t, const std::string&)
or
void attach(const std::list<std::tuple<std::istream&, std::string, content_type_t>>&) ?

[niclar]

@karastojko attach(const std::istream&, const std::string&, media_type_t, const std::string&) seems broken in my use case below.
void attach(const std::list<std::tuple<std::istream&, std::string, content_type_t>>&) however seems to work. Thanks.

msg.content_type(mailio::mime::content_type_t(mailio::mime::media_type_t::TEXT, "html", "utf-8"));
msg.content(content);
msg.content_transfer_encoding(mailio::mime::content_transfer_encoding_t::QUOTED_PRINTABLE);

//msg.attach(attachmentStream, attachmentName, mailio::mime::media_type_t::TEXT, "csv"); <--- destroys the content encoding above
msg.attach({{attachmentStream, attachmentName, mailio::mime::content_type_t(mailio::mime::media_type_t::TEXT, "csv", "utf-8")}});    

[karastojko]

@niclar
The attach(const std::istream&, const std::string&, media_type_t, const std::string&) is broken and will be deprecated. This PR is not closed since it contains the vcpkg port which will be included.

[niclar]

@karastojko re vcpkg. -I did a local one a while back. -Can you expose static lib building something along the line of (or should I create a PR) ? ;

vcpkg_configure_cmake( SOURCE_PATH ${SOURCE_PATH} PREFER_NINJA OPTIONS -DMAILIO_BUILD_SHARED_LIBRARY:BOOL=OFF)

[niclar]

-ah I see that this branch may address that as well.
Awaiting the merge then. Great lib.

[david-antiteum]

@niclar have a look at #39

[david-antiteum]

OK, as the issue is fixed using void attach(const std::list<std::tuple<std::istream&, std::string, content_type_t>>&) I will add a new PR just for vcpkg issues.