Add proxy layer to calculate content type and disposition headers

internal/headers/content_headers.go

internal/headers/headers.go

internal/headers/headers_test.go

internal/senddata/contentprocessor/contentprocessor.go

internal/senddata/contentprocessor/contentprocessor_test.go

internal/senddata/injecter.go

internal/senddata/senddata.go

internal/senddata/writer_test.go

internal/sendfile/sendfile.go

internal/sendfile/sendfile_test.go

internal/testhelper/testhelper.go

testdata/audio.mp3

testdata/file.bmpr

testdata/file.ipynb

testdata/file.pdf

testdata/file.rdoc

testdata/file.sketch

testdata/file.stl

testdata/file.swf

testdata/forgedfile.png