Standardize on contentType or mimeType but not both

[ian]

Feature Description

There's a strange translation happening between UI and core messages, which the params for type of media don't line up.

The UI portion uses a param contentType:

ai/packages/ui-utils/src/types.ts

Line 36 in 9495438

contentType?: string;

And core message uses mimeType:

ai/packages/ai/core/prompt/content-part.ts

Line 54 in 9495438

mimeType?: string;

Because of this translation it seems to be getting lost during message persistence. Might be a lot easier to just use the same param across both.

Use Cases

After calling submit({ experimental_attachments: [...] }) on useChat, the stream/generate function calls onFinish will have lost the content type since the params are misaligned.

Additional context

"ai": 4.0.25
"@ai-sdk/openai": 1.0.13
"@ai-sdk/react": 1.0.13