Allow customisation of preview generation (#673)

* Add generate-specific options
* Include no-cache in preview response

pkg/utils/file.go

@@ -3,13 +3,15 @@ package utils
 import (
 	"archive/zip"
 	"fmt"
-	"github.com/h2non/filetype"
-	"github.com/h2non/filetype/types"
 	"io/ioutil"
 	"math"
+	"net/http"
 	"os"
 	"os/user"
 	"path/filepath"
+
+	"github.com/h2non/filetype"
+	"github.com/h2non/filetype/types"
 )
 
 // FileType uses the filetype package to determine the given file path's type
@@ -219,3 +221,11 @@ func GetParent(path string) *string {
 		return &parentPath
 	}
 }
+
+// ServeFileNoCache serves the provided file, ensuring that the response
+// contains headers to prevent caching.
+func ServeFileNoCache(w http.ResponseWriter, r *http.Request, filepath string) {
+	w.Header().Add("Cache-Control", "no-cache")
+
+	http.ServeFile(w, r, filepath)
+}