first (BIG) commit

helpers/helpers.go

@@ -0,0 +1,82 @@
+// Package helpers implements simple functions used across the application.
+package helpers
+
+import (
+	"mime/multipart"
+	"net/http"
+	"os"
+	"strings"
+)
+
+// allowedContentTypes contains all allowed content types
+// by the application.
+var allowedContentTypes = []string{
+	"application/pdf",
+	"text/html",
+	"application/octet-stream",
+	"application/msword",
+	"application/zip",
+	"application/vnd.openxmlformats-officedocument.wordprocessingml.document",
+	"multipart/form-data",
+}
+
+// GetMatchingContentType parses a content type and tries to find a match
+// with our allowed content types. If no match found, returns an empty string.
+func GetMatchingContentType(contentType string) string {
+	for _, allowedContentType := range allowedContentTypes {
+		if i := strings.IndexRune(contentType, ';'); i != -1 {
+			contentType = contentType[0:i]
+		}
+
+		if contentType == allowedContentType {
+			return allowedContentType
+		}
+	}
+
+	return ""
+}
+
+// getMatchingContentTypeForFile is a simple wrapper of GetMatchingContentType
+// function. It adds a condition for "multipart/form-data" content type, which
+// is not an allowed content type for a file.
+func getMatchingContentTypeForFile(contentType string) string {
+	matchingContentType := GetMatchingContentType(contentType)
+	if matchingContentType == "multipart/form-data" {
+		return ""
+	}
+
+	return matchingContentType
+}
+
+// DetectMultipartFileContentType sniffs the content type of a file from
+// form data.
+func DetectMultipartFileContentType(file multipart.File) (string, error) {
+	// only the first 512 bytes are used to sniff the content type.
+	buffer := make([]byte, 512)
+	n, err := file.Read(buffer)
+	if err != nil {
+		return "", err
+	}
+
+	// resets the read pointer.
+	file.Seek(0, 0)
+
+	// using n if size of buffer < 512 bytes.
+	return getMatchingContentTypeForFile(http.DetectContentType(buffer[:n])), nil
+}
+
+// DetectFileContentType sniffs the content type of a file.
+func DetectFileContentType(file *os.File) (string, error) {
+	// only the first 512 bytes are used to sniff the content type.
+	buffer := make([]byte, 512)
+	n, err := file.Read(buffer)
+	if err != nil {
+		return "", err
+	}
+
+	// resets the read pointer.
+	file.Seek(0, 0)
+
+	// using n if size of buffer < 512 bytes.
+	return getMatchingContentTypeForFile(http.DetectContentType(buffer[:n])), nil
+}