File type != MIME-Type problem

[tmberthold]

If a file is uploaded or referenced as an external file, the user has the option to specify the file type in a text field, or it will be pre-filled automatically.

Problem:
This file type information is also used as MIME type.

Would work if the user set the file type manually to the appropriate MIME type, e.g. for a xyz.xml-file instead of the autmatically filled xml he has to enter/the UI has to prefill application/xml. Then a file with the extension .xml will be downloaded instead of a stream and interpreted as such xml-file.

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Not working, data will be received as stream (automatically set):

Causes follwing DSC-log:
Could not resolve media type. Return data as stream. [exception=(Invalid mime type "xml": does not contain '/')]

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Working (manually edited field content by me):