feat: add multer middleware for handling multipart/form-data requests

[Tanmay-008]

Summary

This PR introduces a dynamic file upload middleware in the Motia framework to correctly handle multipart/form-data requests.

Earlier, Motia did not process multipart requests — meaning file uploads via API routes were ignored or caused errors.

This update solves that by integrating Multer as an on-demand middleware that activates only when a request contains multipart/form-data.
It ensures that uploaded files are processed, parsed, and stored at user-defined destinations according to the fileUploadSchema configuration for each step.

Related Issues

fixes #804

Type of Change

[ ] New feature

Checklist

• I have read the CONTRIBUTING.md
• My code follows the code style of this project
• I have added tests where applicable
• I have tested my changes locally
• I have linked relevant issues
• I have added screenshots for UI changes (if applicable)

Screenshots (if applicable)

Additional Context

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[rohitg00]

Thanks for this PR, I'll let @sergiofilhowz and @ytallo take over this, but can you please ignore committing the pnpm-lock file for this? @Tanmayshi thanks

[Tanmay-008]

@rohitg00 , Thanks for reviewing! Sure, I’ll avoid committing the pnpm-lock.yaml file next time. It got updated automatically because I installed multer for handling file uploads.

[sergiofilhowz]

Unfortunately as much as i like the implementation, this doesn't quite work in cloud environment, especially with AWS Lambdas where we don't have full control over the filesystem

[sergiofilhowz]

Thanks for this PR, I'll let @sergiofilhowz and @ytallo take over this, but can you please ignore committing the pnpm-lock file for this? @Tanmayshi thanks

this one is necessary since he's adding a new dependency (multer)

@Tanmayshi let's hold this for a bit, there's another PR that addresses long data through process that we could use in this