FormData error

[prk-Jr]

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Issue Info

Can not upload multiple files via FormData in Dio 4.0.0 but easily uploadable in 3.0.9

Info	Value
Platform Name	flutter
Platform Version	2.2.1
Dio Version	4.0.0
Android Studio / Xcode Version	e.g. Android Studio 3.3.2 / Xcode 10.2.1
Repro rate	e.g. all the time (100%) / sometimes x% / only once
Repro with our demo prj	e.g. does it happen with our demo project?
Demo project link	e.g. link to a demo project that highlights the issue

Issue Description and Steps

Please fill in the detailed description of the issue (full output of any stack trace, compiler error, ...) and the steps to reproduce the issue.

[stale]

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