can we import excel files and make concurrent changes by multiple users at same time in wiki
Attach the files - no, as the attachments are distinct editable objects in their own right and have to be downloaded and uploaded when being changed. The office connectors can make that a bit more transparent, but it's still not concurrent.
Importing the files so that they become Confluence pages - yes, although you need to be on Cloud to have that feature (it's in Confluence 6, so not there for Server yet)
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