Remove temporary files also on success

* Up to now, the temporary files on the filesystem were only removed
  when an error occured - resulting in an ever growing docker container.
  This changeset makes sure that the temporary files also get removed on
  success.

see #26

src/api/api.go

@@ -168,6 +168,8 @@ func send(c *gin.Context, attachmentTmpDir string, signalCliConfig string, numbe
 		c.JSON(400, gin.H{"error": err.Error()})
 		return
 	}
+
+	cleanupTmpFiles(attachmentTmpPaths)
 	c.JSON(201, nil)
 }