diff --git a/github/github.go b/github/github.go
index f92b1eb..0e8c1ce 100644
--- a/github/github.go
+++ b/github/github.go
@@ -32,7 +32,8 @@ const (
 	headerRateRemaining = "X-RateLimit-Remaining"
 	headerRateReset     = "X-RateLimit-Reset"
 
-	mediaTypeV3 = "application/vnd.github.v3+json"
+	mediaTypeV3      = "application/vnd.github.v3+json"
+	defaultMediaType = "application/octet-stream"
 )
 
 // A Client manages communication with the GitHub API.
@@ -166,7 +167,7 @@ func (c *Client) NewRequest(method, urlStr string, body interface{}) (*http.Requ
 // NewUploadRequest creates an upload request. A relative URL can be provided in
 // urlStr, in which case it is resolved relative to the UploadURL of the Client.
 // Relative URLs should always be specified without a preceding slash.
-func (c *Client) NewUploadRequest(urlStr string, reader io.Reader, contentType string) (*http.Request, error) {
+func (c *Client) NewUploadRequest(urlStr string, reader io.Reader, size int64, mediaType string) (*http.Request, error) {
 	rel, err := url.Parse(urlStr)
 	if err != nil {
 		return nil, err
@@ -177,9 +178,13 @@ func (c *Client) NewUploadRequest(urlStr string, reader io.Reader, contentType s
 	if err != nil {
 		return nil, err
 	}
+	req.ContentLength = size
 
+	if len(mediaType) == 0 {
+		mediaType = defaultMediaType
+	}
+	req.Header.Add("Content-Type", mediaType)
 	req.Header.Add("Accept", mediaTypeV3)
-	req.Header.Add("Content-Type", contentType)
 	req.Header.Add("User-Agent", c.UserAgent)
 	return req, nil
 }
diff --git a/github/github_test.go b/github/github_test.go
index 76d943e..835592b 100644
--- a/github/github_test.go
+++ b/github/github_test.go
@@ -85,6 +85,18 @@ func testURLParseError(t *testing.T, err error) {
 	}
 }
 
+func testBody(t *testing.T, r *http.Request, want string){
+b, err := ioutil.ReadAll(r.Body)
+		if err != nil {
+			t.Errorf("Unable to read body")
+		}
+		str := string(b)
+		if want != str {
+			t.Errorf("Body = %s, want: %s", str, want)
+		}
+	}
+
+
 // Helper function to test that a value is marshalled to JSON as expected.
 func testJSONMarshal(t *testing.T, v interface{}, want string) {
 	j, err := json.Marshal(v)
diff --git a/github/repos_releases.go b/github/repos_releases.go
index 0477ada..bfeafd8 100644
--- a/github/repos_releases.go
+++ b/github/repos_releases.go
@@ -6,8 +6,11 @@
 package github
 
 import (
+	"errors"
 	"fmt"
-	"io"
+	"mime"
+	"os"
+	"path/filepath"
 )
 
 // RepositoryRelease represents a GitHub release in a repository.
@@ -209,16 +212,26 @@ func (s *RepositoriesService) DeleteReleaseAsset(owner, repo string, id int) (*R
 }
 
 // UploadReleaseAsset creates an asset by uploading a file into a release repository.
+// To upload assets that cannot be represented by an os.File, call NewUploadRequest directly.
 //
 // GitHub API docs : http://developer.github.com/v3/repos/releases/#upload-a-release-asset
-func (s *RepositoriesService) UploadReleaseAsset(owner, repo string, id int, opt *UploadOptions, reader io.Reader, contentType string) (*ReleaseAsset, *Response, error) {
+func (s *RepositoriesService) UploadReleaseAsset(owner, repo string, id int, opt *UploadOptions, file *os.File) (*ReleaseAsset, *Response, error) {
 	u := fmt.Sprintf("repos/%s/%s/releases/%d/assets", owner, repo, id)
 	u, err := addOptions(u, opt)
 	if err != nil {
 		return nil, nil, err
 	}
 
-	req, err := s.client.NewUploadRequest(u, reader, contentType)
+	stat, err := file.Stat()
+	if err != nil {
+		return nil, nil, err
+	}
+	if stat.IsDir() {
+		return nil, nil, errors.New("The asset to upload can't be a directory")
+	}
+
+	mediaType := mime.TypeByExtension(filepath.Ext(file.Name()))
+	req, err := s.client.NewUploadRequest(u, file, stat.Size(), mediaType)
 	if err != nil {
 		return nil, nil, err
 	}
diff --git a/github/repos_releases_test.go b/github/repos_releases_test.go
index bfb9c26..6ae08b1 100644
--- a/github/repos_releases_test.go
+++ b/github/repos_releases_test.go
@@ -9,8 +9,8 @@ import (
 	"encoding/json"
 	"fmt"
 	"net/http"
+	"os"
 	"reflect"
-	"strings"
 	"testing"
 )
 
@@ -207,15 +207,20 @@ func TestRepositoriesService_UploadReleaseAsset(t *testing.T) {
 
 	mux.HandleFunc("/repos/o/r/releases/1/assets", func(w http.ResponseWriter, r *http.Request) {
 		testMethod(t, r, "POST")
-		testHeader(t, r, "Content-Type", "application/zip")
+		testHeader(t, r, "Content-Type", "text/plain; charset=utf-8")
+		testHeader(t, r, "Content-Length", "12")
 		testFormValues(t, r, values{"name": "n"})
+		testBody(t, r, "Upload me !\n")
 
 		fmt.Fprintf(w, `{"id":1}`)
 	})
 
 	opt := &UploadOptions{Name: "n"}
-	data := strings.NewReader("data")
-	asset, _, err := client.Repositories.UploadReleaseAsset("o", "r", 1, opt, data, "application/zip")
+	file, err := os.Open("testdata/upload.txt")
+	if err != nil {
+		t.Errorf("Unable to open file testdata/upload.txt")
+	}
+	asset, _, err := client.Repositories.UploadReleaseAsset("o", "r", 1, opt, file)
 	if err != nil {
 		t.Errorf("Repositories.UploadReleaseAssert returned error: %v", err)
 	}
diff --git a/github/testdata/upload.txt b/github/testdata/upload.txt
new file mode 100644
index 0000000..5863dc7
--- /dev/null
+++ b/github/testdata/upload.txt
@@ -0,0 +1 @@
+Upload me !