Merge pull request #2511 from thiagomini/patch-4

docs: add a warning for FileTypeValidator

Author: kamilmysliwiec

content/techniques/file-upload.md

@@ -120,6 +120,8 @@ export abstract class FileValidator<TValidationOptions = Record<string, any>> {
 - `MaxFileSizeValidator` - Checks if a given file's size is less than the provided value (measured in `bytes`)
 - `FileTypeValidator` - Checks if a given file's mime-type matches the given value. 
 
+> warning **Warning** The [FileTypeValidator](https://github.com/nestjs/nest/blob/master/packages/common/pipes/file/file-type.validator.ts) class uses a naive implementation to verify the incoming file's type. Consider using a custom implementation (like checking the file's [magic number](https://www.ibm.com/support/pages/what-magic-number)) if your app requires a safer solution.
+
 To understand how these can be used in conjunction with the beforementioned `FileParsePipe`, we'll use an altered snippet of the last presented example:
 
 ```typescript