fix(pay-slip)[backend]: add file type validation in UploadFile handler

[MohamadNazik]

Summary

This PR fixes the issue #30 by adding file type validation to the upload endpoint. The API now restricts uploads to supported formats to prevent invalid file associations.

Changes

• Validation: Restricted uploads to .pdf, .png, .jpg, and .jpeg (case-insensitive).
• Error Handling: Non-supported types now return a 400 Bad Request.
• Docs: Updated the OpenAPI spec to reflect supported formats.

Verification

• Verified supported formats upload correctly.
• Verified unsupported formats are rejected with 400 error.

Closes #30

[gemini-code-assist]

Summary of Changes

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This pull request enhances the robustness of the pay-slip upload functionality by introducing strict file type validation. It ensures that only specified document and image formats can be uploaded, preventing potential issues from invalid file associations and improving data integrity.

Highlights

• File Type Validation: Implemented server-side validation for uploaded files, restricting them to .pdf, .png, .jpg, and .jpeg formats (case-insensitive).
• Error Handling: Configured the API to return a 400 Bad Request status for any upload attempts with unsupported file types.
• API Documentation Update: Updated the OpenAPI specification to accurately reflect the newly enforced supported file formats for uploads.

Changelog

• pay-slip-app/backend/api/openapi.yaml
  • Updated the description for the file upload endpoint to list the explicitly allowed file extensions.
• pay-slip-app/backend/internal/constants/constants.go
  • Added a new global variable AllowedExtensions containing a slice of strings for the permitted file extensions.
• pay-slip-app/backend/internal/handlers/payslip_handlers.go
  • Imported the path/filepath package to handle file extension extraction.
  • Implemented logic within the UploadFile handler to extract the file extension and compare it against the constants.AllowedExtensions list.
  • Added an early return with an http.StatusBadRequest error if the uploaded file's extension is not among the allowed types.

Activity

• The author verified that supported file formats can be uploaded successfully.
• The author verified that unsupported file formats are correctly rejected with a 400 error.

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[gemini-code-assist]

Code Review

This pull request successfully implements file type validation for uploads, addressing issue #30. The changes include updating the OpenAPI specification to reflect the supported formats, centralizing the list of allowed extensions in constants.go, and adding robust validation logic in the UploadFile handler. The validation correctly performs a case-insensitive check on file extensions and returns an appropriate 400 Bad Request for unsupported types. This is a good improvement for the application's security and data integrity.

[ginaxu1]

Extension-based validation can still be spoofed. To make this more robust: use a centralized map for lookups (remove the switch/case) + add content-sniffing support. Here's how you can rewrite this mime_utils.go

// mimeMapping provides a single source of truth for allowed extensions and their canonical MIME types
var mimeMapping = map[string]string{
	".pdf":  "application/pdf",
	".png":  "image/png",
	".jpg":  "image/jpeg",
	".jpeg": "image/jpeg",
}

// Check the filename extension and actual content (magic bytes)
// Returns MIME type and a boolean indicating if the file is valid
func ValidateFile(filename string, content []byte) (string, bool) {
	ext := strings.ToLower(filepath.Ext(filename))
	
	// Validate Extension against our known allowed list
	canonicalMime, supported := mimeMapping[ext]
	if !supported {
		return "", false
	}

	// Validate actual content for security
	// http.DetectContentType uses the first 512 bytes to determine the MIME type.
	detectedMime := http.DetectContentType(content)

	// Clean detected MIME (remove charset info if present)
	if parts := strings.Split(detectedMime, ";"); len(parts) > 0 {
		detectedMime = parts[0]
	}

	// Make sure the extension matches the actual content type
	// This prevents a user from uploading a .exe renamed to .pdf
	if detectedMime != canonicalMime {
		return "", false
	}

	return canonicalMime, true
}

// GetMimeByExtension is a helper for cases where only the filename is available
func GetMimeByExtension(filename string) (string, bool) {
	ext := strings.ToLower(filepath.Ext(filename))
	mime, allowed := mimeMapping[ext]
	return mime, allowed
}