QFX files are Quicken OFX (Web Connect) Format files containing details about transactions:
Download and install the Transactions app
Start Transactions and locate a QFX file
Review transactions
Click Save to save a CSV file and then open it in Excel (or other spreadsheet application) or click Copy to clipboard and then switch to Excel and click Paste there
Bank (Checking/Savings/Credit Line)
Credit Card
Investment
Here is a short video (1 min) showing whole process of converting a QFX file to CSV format with QFX2CSV (replaced by the Transactions app):