Update added support for big excel files

[ThomasObenaus]

Adds support for big excel files

By adding the possibility to configure the amount of local headers checked

There the problem can be (depending on the library generating the file) that the headers are more than 10 and the relevant ones for identifying it as an excel file are at place 11 or above.

Reason for the Change

We have some bigger excel documents (which I can't share due to compliance restrictions) that aren't detected correctly.
After analyzing the problem it turned out that it is not enough for those kind of documents to check only the first 10 local headers.

How we use it with that change

// increase the read limit (default is 3072) and the amount of MSO file headers checked to support big excel files as well
mimetype.SetLimit(20480)
mimetype.SetMaxMSOFileHeaders(100)
detectedMimeType := mimetype.Detect(data)

[gabriel-vasile]

I'm concerned not many users of the library are familiar MSO file headers and will / can be bothered to learn about it.

I think it would be great to fix this issue without adding the SetMaxMSOFileHeaders function. I see 2 steps for this:

First, we can remove the 10 headers limit and search in all available headers. I added this limit, but thinking back on it I'm not sure about it. There already is a limit for the max number of bytes read from input which also limits the number of headers.
Second, we can add more known excel headers. If you'd like, you can add them yourself, or tell me the library used to generate the Excels and I'll have a look.

@ThomasObenaus Do you think this approach will work for you?

[ThomasObenaus]

@gabriel-vasile I adjusted my PR according to your request.

[ThomasObenaus]

Sadly I can't tell you how the file was created since it was uploaded by a customer.
And as I said I can't provide the file itself either.

But I can provide (at least) the file structure of that xlsx document:

I assume there where just too much files in the customXML folder. So the search never found any of the xlsxSigFiles within the first 10 files.

[gabriel-vasile]

Thank you for your contribution @ThomasObenaus!