Your metadata is not visible to the Session Technology Foundation or Oxen File Server when sending attachments. However, if you want to prevent your chat partner from seeing file metadata, consider stripping the metadata before sending the file. Here is a how-to article.
Session uses onion routing to hide your IP address when uploading or downloading attachments from the Oxen File Server. All attachments stored on the server are encrypted, and can only be decrypted by your chat partner(s) — so the Oxen File Server cannot view your files’ metadata.
In addition to this, EXIF metadata is stripped when sending a file (except videos) sent from Desktop (but not mobile platforms).