[PowerShell] Support multiple files in multipart/form-data POST (new functionality + fix)

[condorcorde]

api.mustache was modified again in order to support the Dradis API that allows multiple files to be submitted in the same multipart/form-datarequest.

En passant, a required cast to System.IO.FileInfo that was lost in a ghost commit has been reintroduced.

Here are examples of requests generated during the tests of this PR, with one and two files:

$ New-DRDAttachment -DradisProjectId 166 -NodeId 4440 -Files a.png

POST /pro/api/nodes/4440/attachments HTTP/2
[...]
Content-Type: multipart/form-data; boundary="fcf39142-ebb8-46c1-9dda-1827e6685617"
Content-Length: 650

--fcf39142-ebb8-46c1-9dda-1827e6685617
Content-Disposition: form-data; name="files[]"; filename=a.png; filename*=utf-8''a.png
Content-Type: application/octet-stream

�PNG
[...]
--fcf39142-ebb8-46c1-9dda-1827e6685617--

$ New-DRDAttachment -DradisProjectId 166 -NodeId 4440 -Files a.png,b.png

POST /pro/api/nodes/4440/attachments HTTP/2
[...]
Content-Type: multipart/form-data; boundary="c5d81892-2bec-4150-b31a-c638e0f0eb19"
Content-Length: 1279

--c5d81892-2bec-4150-b31a-c638e0f0eb19
Content-Disposition: form-data; name="files[]"; filename=a.png; filename*=utf-8''a.png
Content-Type: application/octet-stream

�PNG
[...]
--c5d81892-2bec-4150-b31a-c638e0f0eb19
Content-Disposition: form-data; name="files[]"; filename=b.png; filename*=utf-8''b.png
Content-Type: application/octet-stream

�PNG
[...]
--c5d81892-2bec-4150-b31a-c638e0f0eb19--

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[wing328]

appveyor tests passed via https://ci.appveyor.com/project/WilliamCheng/openapi-generator/builds/50472428