Enable `.mtl` File Uploads

[SableRaf]

Increasing Access

Allowing .mtl file uploads in the p5.js Web Editor would ensure feature consistency between the editor and p5.js itself, reducing potential confusion for users.

Feature enhancement details

The p5.js Web Editor currently blocks the upload of .mtl files with a "you can't upload files of this type" error, despite .mtl file support being available in p5.js since version 1.9.2 (see processing/p5.js#6710). This discrepancy limits users' ability to fully utilize p5.js's 3D capabilities within the Web Editor.

[lindapaiste]

It needs to be added here and I think that's it? https://github.com/processing/p5.js-web-editor/blob/c4de2669ad4df5b0e99927a4dff8c7394d8f0744/server/utils/fileUtils.js