[bugfix] Wrap media in read closer (#941)

* use readcloser for content.Content

* call media postdata function no matter what

* return a readcloser from data func

* tidy of logic of readertostore

* fix whoopsie

internal/media/processingemoji.go

@@ -193,24 +193,22 @@ func (p *ProcessingEmoji) store(ctx context.Context) error {
 		return nil
 	}
 
-	// execute the data function to get the reader out of it
-	reader, fileSize, err := p.data(ctx)
+	// execute the data function to get the readcloser out of it
+	rc, fileSize, err := p.data(ctx)
 	if err != nil {
 		return fmt.Errorf("store: error executing data function: %s", err)
 	}
 
 	// defer closing the reader when we're done with it
 	defer func() {
-		if rc, ok := reader.(io.ReadCloser); ok {
-			if err := rc.Close(); err != nil {
-				log.Errorf("store: error closing readcloser: %s", err)
-			}
+		if err := rc.Close(); err != nil {
+			log.Errorf("store: error closing readcloser: %s", err)
 		}
 	}()
 
 	// extract no more than 261 bytes from the beginning of the file -- this is the header
 	firstBytes := make([]byte, maxFileHeaderBytes)
-	if _, err := reader.Read(firstBytes); err != nil {
+	if _, err := rc.Read(firstBytes); err != nil {
 		return fmt.Errorf("store: error reading initial %d bytes: %s", maxFileHeaderBytes, err)
 	}
 
@@ -242,7 +240,7 @@ func (p *ProcessingEmoji) store(ctx context.Context) error {
 	p.emoji.ImageContentType = contentType
 
 	// concatenate the first bytes with the existing bytes still in the reader (thanks Mara)
-	readerToStore := io.MultiReader(bytes.NewBuffer(firstBytes), reader)
+	readerToStore := io.MultiReader(bytes.NewBuffer(firstBytes), rc)
 
 	var maxEmojiSize int64
 	if p.emoji.Domain == "" {