There are some web services that accept uploading files via the POST method, only if the files are in Base64 string format. In order to achieve that, the simplest way is to use the "Run Powershell Script" Action, inserting the following command: 

[convert]::ToBase64String((Get-Content -path "your_file_path" -Encoding byte))

The Powershell output will be a text type variable with the representation of the specified file in Base64 format.

Note that the file path can also be put as a variable in the "Run Powershell Script" Action.