The group admin or group admins of a group can release files for editing.
By default, all files uploaded to the cloud cannot be edited by every group member.
This means that the respective group admin must always release the corresponding file for editing manually after the upload.
With the chat open, the creator of the file can edit it even if he/she is not a group admin. However, he/she can never share the file with other group members for editing. Only the group admin can set this.