There are three different methods for uploading file content to Transifex:

Using Content-Type application/vnd.api+json the content must be JSON serializable.

For content that is directly JSON serializable you can dump the content of the file directly and set the content_encoding attribute to text.
For any content, even if it's not JSON serializable (eg .xlsx, .idml or .docx file extensions), you can encode the file using the base64 encoding and set the content_encoding attribute to base64.
Using Content-Type multipart/form-data the content can be any raw file, and can be send without further processing.