Is there a setting that prevents images and text files to be imported when I ask a folder to be imported in MC?
Yes of course. Under Auto Import, select folder that you import, then click the Edit button. With the Mode set to "Import this folder and all child folders", check the file types that are being imported. Make sure image and text documents aren't selected. Also make sure that "Ignore special files (folder.jpg, etc.)" is ticked. You can also set up an item with the Mode set to "Exclude this folder and all child folders", and select the file types that you want to exclude.
Even when you import a single folder, you can uncheck any file types that you don't want to import, and MC will remember those settings.
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But what about images? There is no set rule (in my practice over the years) that specifies how the cover art is named within a folder, not whether it is in a subfolder of the folder.
That is easy as well. At least with my setup, I don't store any image, text, or other files under my Music directory that I want MC to import. Cover Art is not imported into MC, or isn't supposed to be. It is just used by MC in association with other file types, for display.
So I just navigate to "Audio > Files" which is a panes view, select any unwanted files in the Library under File Type, such as JPGs, and delete them all from the Library (not from the drive though). If you are concerned about where these image files are actually stored, and need that information to make a decision, just drill down in the Location item on the left to the folder on the drive that holds your music. But I just used the "All Locations" group to clean up some old images I had unintentionally imported into MC on a test Library. That covers any files import with a [Media Type] of "Audio".
Repeat for Video.
Then turn on the Image and Data features, and check them in the same way as above. The only images MC should know about, other than Cover Art which isn't shown as items in the Library, should be actual images you have deliberately imported. Which means there should be none.
Of course, you would just be deleting those images from the Library, not from the disk, unless you do also want to delete them from the disk, permanently.
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There is another way that is somewhat easier, but a little bit more dangerous.Just navigate to "Drive & Devices > Explorer" and then drill down to, for example, your base music directory. Then tick the box up the top "Show Only Files In Library", and then "Show Files In Subfolders".
Now you will have a view of all files in the Library under your base music directory. You can now use the search function to find any files that do not belong. So you could search for specific file types that shouldn't be in your Library, such as jpg, png and txt files, using the search "[File Type]=jpg,png,txt". You could also search for any files that do not match the Audio files types that you own, for example using the search "-[File Type]=flac,mp3,m4a", which says 'Show any files that are not flac, mp3, or m4a files. Perhaps scroll through the list to check the results, before selecting any unwanted files and deleting them from the Library (not the disk).
Just be careful with using the MC Explorer functionality. Make sure you do tick the "Show Only Files In Library" box before deleting anything. If you untick that box, MC will show you all files and allow you to do normal things with files, like delete or move then, even though those files aren't imported into MC. Even OS files! MC actually temporarily imports all those files in order to display them, so it can take a while to show the contents of large folders. MC deletes those temporary records once you finish using MC Explorer.
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In addition to that, there is the problem that some folders (quite a few) contain subfolders, such as "Art", "Logs" or whatever.
These subfolders are not moved.
Well, Swiv3d has confirmed that is an issue. I don't use sub-folders to store anything, but I could see how it may be a problem, particularly if the folders and files are being used with the MC "Extras" functionality. I haven't seen anything about sub-folders being moved using the "Move all non-imported files in folder with selected media files". I might have to have a play with that.
You will probably need to manage those additional sub-folders manually.