I was able to read a csv file from attachment record, using below apex code:
Attachment attachmentRecord = [Select Id,Body,ContentType from Attachment WHERE ParentId = <Parent Record Id>];
csvAsString = attachmentRecord.Body.toString();
csvFileLines = csvAsString.split('\n');
I would assume this is because you did not check the Include column headers box on export, so it is assuming that the top row is just another row and not the headers for your data.
The above is from a manual UI export, You can also find it in a Data Extract Activity (in Automation Studio) as Has Column Headers in the Configuration step. Like below:
Use the Text to Columns feature in Excel to divide the data into separate columns before you do your import.
As a general rule, it is almost always easier to perform data transformations before you move the data into Salesforce.