Add support for images and pdf files

[Yahiewi]

This PR fixes #292 by fixing the way attachments are read and sent to the AI.

Main code changes:

• In the sendMessage function, enhancedMessage is now declared as UserContent and attachmentContents is now replaced by { textContents, binaryParts }
• Changed _processAttachments return signature changed from Promise<string[]> to Promise<{ textContents: string[] binaryParts: Array<ImagePart | FilePart>}> and replaced const contents with textContents: string[] and binaryParts : Array<ImagePart | FilePart>
• Defined:
  -imageMimeTypes : .png, .jpg, .jpeg, .gif, .webp, .bmp (which is sent as ImagePart)
  -fileMimeTypes : .pdf (which is sent as FilePart)
• Defined _readBinaryAttachment function to take in both images and PDFs and return their base64 content.

Screencast.from.2026-04-02.10-51-56.mp4

[Yahiewi]

GIFs are not/only partially supported by all AIs as input. Claude, for example can see them as still images, Mistral however produces an 'Error generating response' with the following console log: AI_APICallError: Image is an animated GIF.