fix(core): marking files as pipeable

[lucavb]

The two decorators @uploadedfile and @uploadedfiles have been marked as pipeable,
however this had no affect as this method was preventing their execution.

PR Checklist

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• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
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PR Type

What kind of change does this PR introduce?

[X] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Other... Please describe:

What is the current behavior?

Currently, the interface for @UploadedFile and @UploadedFiles describes the possibility of adding pipes to somehow transform the file object. However, this does not work at the moment since the isPipeable method does not accept FILE and FILES as "pipeable".

Issue Number: #4752 (original feature request) / #6344 (comment) (specific bug targeted by this PR)

What is the new behavior?

The pipes are actually run and can alter the file object and the files array.

Does this PR introduce a breaking change?

[ ] Yes
[X] No

Other information

Personally I would tend to refactor that many comparisons of a value to a set so that I can call .has(). If that is something you want, then I can update the PR

[coveralls]

Pull Request Test Coverage Report for Build 4aad8c0e-8e3f-48bc-8ccb-1c41744358ce

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 94.621%

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Totals
Change from base Build 77925e0e-a374-4cbf-9a84-98f285ee9e40:	0.0%
Covered Lines:	5119
Relevant Lines:	5410

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💛 - Coveralls

[kamilmysliwiec]

LGTM, it will be added in the next release