Add Accept header negotiation to relevant API endpoints (#337)

* start centralizing negotiation logic for API

* swagger document nodeinfo endpoint

* go fmt

* document negotiate function

* use content negotiation

* tidy up negotiation logic

* negotiate content throughout client api

* swagger

* remove attachment on Content

* add accept header to test requests

internal/api/client/fileserver/servefile_test.go

@@ -123,6 +123,7 @@ func (suite *ServeFileTestSuite) TestServeOriginalFileSuccessful() {
 	recorder := httptest.NewRecorder()
 	ctx, _ := gin.CreateTestContext(recorder)
 	ctx.Request = httptest.NewRequest(http.MethodGet, targetAttachment.URL, nil)
+	ctx.Request.Header.Set("accept", "*/*")
 
 	// normally the router would populate these params from the path values,
 	// but because we're calling the ServeFile function directly, we need to set them manually.