Cast tuple of filenames to list to improve error handling

[mscheltienne]

@sappelhoff Is this better? c.f. #12843 (comment)

Now:

# %%
from pathlib import Path
import numpy as np
import mne

# create a dummy raw
sampling_freq = 1000
times = np.linspace(0, 1, sampling_freq, endpoint=False)
sine = np.sin(20 * np.pi * times)
cosine = np.cos(10 * np.pi * times)
data = np.array([sine, cosine])
info = mne.create_info(
    ch_names=["10 Hz sine", "5 Hz cosine"], ch_types=["misc"] * 2, sfreq=sampling_freq
)
raw = mne.io.RawArray(data, info)

# I want to set raw.filenames
a = Path.cwd() / "IDoNotExist.jpg"

# previous error: TypeError: 'tuple' object does not support item assignment
# new error: FileNotFoundError: File <...>/IDoNotExist.jpg not found.
raw.filenames = (a,)

# this gives the right error: FileNotFoundError: File <...>/IDoNotExist.jpg not found.
raw.filenames = [a]

# this works correctly
#path_to_file_that_exists = ...
#raw.filenames = [path_to_file_that_exists]

# this works, no errors raised (which is ok, because I am using a private attribute to circumvent checks)
raw._filenames = (a,)

[sappelhoff]

thanks a lot!