Unable to send audio file to MCP Server as base64

[IPv6]

I have an private remote transcription MCP server (/sse based), which accepts base64 string with content, it is expected to get content of audio file in base64 encoding. but no matter what i tried - client was never able to send it properly...

I am using Mistral and have filesystem tool enabled and prompt is like "Transcribe audio file /.../text.mp3 into text" - transcribing is what transcription MCP server performs. Tool parameters are hinted for base64, tool description directly asks for base64 - but no luck. All i see is garbage:

How this should be prompted? Any ideas?