Return error of context.File in c.contentDisposition

context.Attachment and context.Inline use context.contentDisposition under the hood.
However, context.contentDisposition does not forward the error of context.File, leading to response 200 OK even when the file does not exist.
This commit forward the return value of context.File to context.contentDisposition to prevent that.

context.go

@@ -509,18 +509,17 @@ func (c *context) File(file string) (err error) {
 	return
 }
 
-func (c *context) Attachment(file, name string) (err error) {
+func (c *context) Attachment(file, name string) error {
 	return c.contentDisposition(file, name, "attachment")
 }
 
-func (c *context) Inline(file, name string) (err error) {
+func (c *context) Inline(file, name string) error {
 	return c.contentDisposition(file, name, "inline")
 }
 
-func (c *context) contentDisposition(file, name, dispositionType string) (err error) {
+func (c *context) contentDisposition(file, name, dispositionType string) error {
 	c.response.Header().Set(HeaderContentDisposition, fmt.Sprintf("%s; filename=%q", dispositionType, name))
-	c.File(file)
-	return
+	return c.File(file)
 }
 
 func (c *context) NoContent(code int) error {