To upload a file to the server, use Blog → Upload Media.... You are then asked to select a file from your disk. The media upload dialog will be displayed:

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If your blog API supports media uploading, you have 2 options here, and if not, there is just one and it's FTP server.

You can preview your desired file here (if preview method available), and enter its name on the server. And then click Upload.

If you want to upload to an FTP server, the absolute path of the folder is required. And you have to enter a URL to use for identifying the link of the file on the server.