If I encrypt the contents of a file through File Explorer in Windows 10 (Properties -> Attributes -> Advanced) and upload it to the internet (cloud, attachment in an email, etc.), will I still be the only one who can view its contents?
The encryption you activate is the EFS (Encrypting File System). It is designed to encrypt files while they reside on your local NTFS formatted hard disk.
The encryption is transparent to you; this means whenever you read or modify the encrypted file, Windows automatically decrypts the file for you so you can work with the original decrypted file. As long as you don't lose the decryption key you don't even recognize that the file is encrypted on your hard disk.
Therefore, as you have uploaded the file to the internet, the file you used for uploading has read the file and Windows has automatically decrypted the file. This means the uploaded file is decrypted and the content is visible to anyone.
Just a rule-of thumb: If you encrypt something and it doesn't ask for a key or a password, then the encryption is always limited to your device (or the encryption software is just a hoax).