Fix/mail connector attachments

[TobiasAdler]

Description

• Added a data driven testcase for different binary attachments in MailConnectorTest and adjusted the testcase for plain text attachment
• Created a new object EmailAttachment which stores the name, input stream and the encoding of an attached file
• Adjusted readMessageContents(), getCharSetForEncoding() and other methods of Email object to work proberly with binary files and the new EmailAttachment object

Fixes #347

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules